B757 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Troubleshooting; Situational Awareness; Distraction; Confusion; Time Pressure
Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Rejected Takeoff; Flight Crew Returned To Gate; General Maintenance Action
Weather; Human Factors
After a two-hour delay for high winds at the airport (above 50 kts); we were finally able to load the airplane and depart the gate for our crossing.  We started engines normally; and obtained and checked our load closeout before we began the short taxi to runway XX.  The wind; by this time; was only gusting to 34 kts and was pretty much straight down the runway. We ran the Before Takeoff Checklist normally with the trim set and verified at 4.4 units.  The tower cleared us for takeoff and the First Officer advanced the thrust levers; as it was his leg. The takeoff proceeded normally until at about 120 kts (V1 was around 150 kts) when we suddenly got a master warning and bell.  I glanced down and saw that we had a configuration warning.  I hesitated for beat and made the decision to abort the takeoff. The abort was normal and since I could see that we were decelerating quickly and would easily make our normal exit; I disengaged the RTO auto brake function to reduce from maximum to normal braking. We exited the runway and began to access the situation. I then saw that the configuration warning was for Stabilizer.  I looked down and was astonished to see the trim was at 8 or 9 nose up units.  I began to suspect a runaway trim; but we had no other warnings.  We began a slow taxi back toward the terminal while the IRO consulted the cooling charts.  We were good to proceed to the gate. During the taxi back; the first officer informed me that he might have been the cause for the configuration warning.  He was using a technique that he said had been pointed out to him by at least a couple of first officers.  He said that he would trim nose up during the takeoff roll so that the rotation pressure would not be so heavy.  Neither the IRO nor I had ever heard of this technique.Our brake cooling time was about an hour.  We had maintenance come out and inspect the trim system and the brakes and everything was fine.  He signed off the log and a couple of hours after the event; we departed. The First Officer trimmed the aircraft nose up during the takeoff roll so lighten the rotation force.  He said that he had used this technique before.  This time; however; he held the trim down and moved us out of the green band thus causing the configuration warning. I am very concerned about this technique and whether or not it is widespread.  I have never noticed any of my FOs trimming during the takeoff run or heard of any such discussion; but this is a dangerous technique; for obvious reasons. I counseled the First Officer against using this technique in the future as it could lead to a tail strike or worse in addition to the configuration warning and abort. I believe my First Officer now fully understands the dangers of this technique.
[Report narrative contained no additional information.]
B757 flight crew reported the First Officer trimmed nose up during takeoff which caused a warning as trim exceeded the limit. The First Officer intended to lighten takeoff rotation effort; but was cautioned his action was poor technique.