In reply to timjones:
> (In reply to nniff)
> [...]
Have you ever done this in real life or just read about it on a dodgy web forum
Fairly frequently - the crux groove of Venom at Tremadog was particularly memorable for the second's failure to make progress without assistance and the large amount of hauling that was required. And no you can't do it if the second is hanging in space, or if there is too much rope drag. It is, however, perfectly possible to give a second a big boost by lifting with your legs - which you cannot do with a direct belay.
By all means faff around with bits of string, but the expedient thing to do is to give a sodding great heave and see if it has the required effect.
To quote Pete Livesey on Footless Crow with Chris Bonington, 'Hang on, Chris. The winch has run out of petrol'