a power - operated tool is fitted with a drive , whose driving shaft can , by means of a connecting coupling whose actuation depends on the reaction torque , be coupled with a driven shaft . the connecting coupling first interrupts transfer of the torque to the driven shaft , so that , in spite of the running drive , no torque can be transferred to the driving shaft . immediately afterwards , the drive is disconnected . as a result of this sequence of disconnections , the motor , which continues running , can no longer have any influence on the work in progress so that the fastener being tightened does not reach the point of plastic deformation .