the mandrel 10 is made up of top sub 12 , upper body 14 , lower body 16 and bottom sub 18 . these pieces are preferably threaded together but may be attached in other ways . more or fewer pieces can be used to define the mandrel 10 . an outer sleeve 20 has a window 22 for each dog 24 that is used . one or more dogs 24 can be used . dogs 24 have tabs 26 at opposed ends , as best seen in fig5 to limit the outward travel of the dogs 24 with respect to window 22 . fig1 a shows the dog 24 in section . in the preferred form of dog 24 , it is generally u - shaped having a pair of inwardly oriented legs 28 and 30 . on the trip into the well surface 32 on dog 24 will encounter an obstacle . on the trip out of the well , surface 34 on dog 24 will encounter an obstacle . sleeve 20 is mounted to slide over mandrel 10 . it is biased uphole by spring 36 that bears on surface 38 of bottom sub 18 . spring 40 bears on surface 42 of top sub 12 and applies an opposing force to sleeve 20 than spring 36 . preferably spring 40 is weaker than spring 36 for reasons that will be explained below . upper body 14 has three grooves 44 , 46 , and 48 . these grooves are deep enough so that when legs 28 and 30 are in them , outer surface 50 of dogs 24 recedes inside of window 22 . in this manner the tool can pass an obstruction going downhole and can be removed after release going uphole . if an obstruction is encountered by surface 32 going in the hole , the spring 40 is compressed as the sleeve 20 and dogs 24 stop downhole motion . continued downhole movement of mandrel 10 not only compresses spring 40 but also positions grooves 44 and 46 in alignment with legs 28 and 30 of dogs 24 to allow them to retract to a position closer to the central axis 52 and preferably within sleeve 20 . at that point the obstruction can be passed and spring 40 can bias the sleeve 20 back into the neutral position shown in fig1 . fig2 shows the legs 28 and 30 getting cammed out of grooves 44 and 46 by the action of spring 40 after the obstruction going downhole is cleared . note that sloping surfaces 52 and 54 facilitate the exit of legs 28 and 30 from grooves 44 and 46 under the return force of the formerly compressed spring 40 . with the obstacle cleared going downhole , the dogs 24 resume the neutral run in position shown in fig1 . defined between the sleeve 20 and the mandrel 10 and best seen in fig3 are an upper fluid reservoir 56 and a lower fluid reservoir 58 . a fill port 60 allows charging the fluid at the surface . thermal and hydrostatic effects in this closed system of interconnected reservoirs are fully compensated by a piston 62 that can be biased by belleville washers 64 , for example , or any other device that is comparable . those skilled in the art will appreciate the benefit of such compensation on the structure of the device especially when it is deployed at great depths and / or high temperature applications . fig4 illustrates this execution of a compensation feature . fig2 b best illustrates other features of this reservoir system . there is a flow restrictor 66 that regulates the flow rate from reservoir 58 into reservoir 56 . there is a check valve 68 that permits a bypass of restrictor 66 when the fluid is flowing in the opposite direction from reservoir 56 to reservoir 58 . a pressure relief device 70 is in line with the restrictor 66 so that when fluid is urged in a direction from reservoir 58 to reservoir 56 there will have to be a rise in the driving pressure to cause such flow to a predetermined level before any flow begins . broadly stated , the fluid system is operative to create a delay as the dogs 24 are in the desired location and a force is applied to the mandrel 10 to create a surface signal for such engagement prior to the release of the dogs 24 from the locating groove ( not shown ). the system serves to allow a reduction of the applied pulling force before release to reduce the slingshot effect from release . when used with the optional pressure relief device 70 the tool can be inverted and can be used to apply a load in a predetermined range on a bha without concern for premature release , such as an offshore drilling application where a heave compensator system is employed . now that the main components have been described , the operation of the tool in various applications will be discussed in more detail . fig1 shows the run in position with the dogs 24 having legs 28 and 30 out of any of the grooves 44 , 46 and 48 . preferably , the dogs 24 are biased into the fig1 position where legs 28 and 30 straddle groove 46 by virtue of spring 36 overpowering spring 40 to move sleeve 20 to the fig1 position . as the tool is brought downhole , an obstacle will first hit surface 32 on dogs 24 . the mandrel 10 will continue downhole as the dogs 24 stop the descent of the sleeve 20 . as grooves 44 and 46 come into alignment with legs 28 and 30 , the dogs 24 will be able to retract sufficiently to allow the tool to continue past the obstacle . the dogs 24 can retract within sleeve 20 as much as necessary to allow the obstacle to be cleared . the advancing of the mandrel 10 with the dogs 24 temporarily stuck on an obstacle , compresses spring 40 . after the obstacle is cleared , spring 40 relaxes to return the tool to the fig1 position from the fig2 position . it should be noted that advancing the mandrel downhole with the dogs 24 stopped by an obstacle will result in sleeve 20 taking dogs 24 against the bias of spring 40 taking the lower end 21 of sleeve 20 away from upper end 23 of sleeve 25 , whose relative movement with respect to the mandrel 10 , at other times , creates movement of fluid between reservoirs 56 and 58 . the amount of this movement to reset the dogs 24 to the fig1 position after clearing the obstacle is also quite short . when the desired depth is reached , the tool is pulled up until the surface 34 engages a desired locating groove downhole . at that point , further upward pulling on the mandrel 10 from the work string ( not shown ) will force fluid from reservoir 58 to reservoir 56 through restrictor 66 . this regulates the rate of movement of mandrel 10 as the force is being applied to give surface personnel the time to notice a signal that the desired groove has been engaged and a force that well exceeds the potential drag force from friction of slip / stick effects on the work string in a deviated wellbore are applied . the rig crew can then actually lower the applied pulling force before the actual release happens to reduce the slingshot effect from the release . release occurs after the mandrel 10 moves a sufficient distance to place grooves 46 and 48 in alignment with legs 28 and 30 to allow the dogs 24 to retract and the tool to be returned to the fig1 position . this occurs because the pulling uphole with the dogs 24 in the locating groove compresses spring 36 as seen in fig3 . retraction of the dogs 24 allows spring 36 to overcome spring 40 and the tool returns to the fig1 position , ready for another cycle . with the use of the optional relief device 70 the surface personnel are assured that a pulling force up to a predetermined level will not initiate the release sequence . hence force can be applied and removed any number of times before there is a release . those skilled in the art will appreciate that the tool can be used in an inverted orientation and function similarly in one application , for example where a range of weight on a bha is desired in a given range without fear of initiating a release sequence . in such an application , rather than a pulling force uphole , a pushing force downhole is applied with the dogs 24 engaged in a receptacle . combining with the use of the optional relief device 70 no fluid flow between reservoirs 56 and 58 can happen until a predetermined force is exceeded . this configuration can be used in offshore drilling in conjunction with heave compensators . those skilled in the art will now appreciate that the described tool can allow applied forces in the order of 100 , 000 or more where the collet designs were more limited to lower applied forces in the order of 40 , 000 pounds or less . these lower limits on the collet designs were sometimes not sufficient to exceed friction and slip / stick effects on the work string in highly deviated holes . the use of a dog structure extending through a window and more specifically a dog design having thick upper and lower ends using legs 28 and 30 accounts at least in part for the ability to apply higher forces to clear obstacles and to test the location of the tool in a desired groove in a specific collar , for example . the use of the check valve 68 allows the tool to quickly find its neutral position after a release so that the test can be quickly repeated , if desired . the use of the restrictor 66 allows more time at the surface to hold a force before release and further allows lowering the applied force after the passage of time but before release to reduce the slingshot effect from release . the pressure relief device 70 allows application of force for any desired time without fear of release if the force is kept at a level where the relief device remains closed . the fluid used on the reservoirs can be a liquid or gas . the compensator 62 is an optional feature . the tool is serviceable in the well in opposed orientations depending on the intended service . although 4 dogs 24 are illustrated one or more such dogs can be used . biasing of springs 36 and 40 can be accomplished by equivalent devices . while the invention has been described with a certain degree of particularity , it is manifest that many changes may be made in the details of construction and the arrangement of components without departing from the spirit and scope of this disclosure . it is understood that the invention is not limited to the exemplified embodiments set forth herein but is to be limited only by the scope of the attached claims , including the full range of equivalency to which each element thereof is entitled .