Well, thanks for all the replies, I've found it most interesting. So, I reckon it's a combination of effects, the wrist position is certainly good, and I can feel the metal shaft twisting ever so slightly when undoing a tight fastener, which I think imparts a bit of extra torque.
It's a German driver, so pretty good quality, but I only ever bought it for the reach (it just fitted in through all the pipery around the hydraulic filter in a C-130 freight bay) never realising quite how good it would be in shifting stubborn screws.
So, if you don't have one, it's worth seeking out a nice, thin, long bladed driver; it will complement all of the others in your kit. Plus, my male pride is still intact, having got my sister out of a pickle!