While dead heading in commuter aircraft I have often noticed that both pilots had their hands on the throttles - and I have seen it in the B737 as well. I think it is a gimmick which is based upon the false notion that the the throttles may slip back and therefore the PNF keeps his hand behind the throttles to prevent this. I personally find it quite distracting to have the PNF fiddling with minute changes of power while my hand is firmly in control of the throttles.
The DC-3 had a throttle friction nut problem in this regard, because unless the PNF tightened the friction nut controlling both throttles - after the PF had set take-off power, then unless the PF had a firm grip on the throttles, they would most definately slip back to idle. For this reason the PF would keep his hand behind the throttles as a back up to the friction nut.
Many years ago, an airman stole a Dakota at a RAAF base in Australia. He had been drinking. The aircraft actually got airborne, but when the airman removed his right hand from the throttles in order to select the undercarriage levers up, the throttles slipped back quickly. Witnesses heard several applications of power and it was presumed that he was re-applying throttle in between attempts to raise the gear. Each time the throttles slipped back quickly and finally the aircraft crashed. It didn't help that it was night and that the artificial horizon was caged..
But in modern aircraft there is no need for a second hand behind the throttles for the duration of the take-off because at least in the 737 there is an internal friction device. If the PNF does keep his hand behind the throttles during the take-off run to V1, there is a grave danger that his fingers may be broken if the PF snaps back the throttles in a sudden decision to abort. It is also annoying to have the PNF hands behind the throttles when simply there is no technical reason for it.
In any case most procedures call for the PNF to adjust the take-off power fine-tuning only up to a maximum of 60 knots, then remove his hand completely. I have witnessed the severe pain that occurred when the PF aborted the take-off in the simulator and lacerated the PNF fingers because he could not get his hand out of the way in time.
[ 23 February 2002: Message edited by: Centaurus ]</p>