If I remember rightly, it involved loosely holding the line in one hand, holding the bag in the other, then clipping a torch (or other reasonably weighted object) to the line in between, then letting the torch drop and playing out the loosely held line. Like a pulley system, hopefully as the torch drops it will result in the spool arriving back in your hand. Then it's simply a matter of winding in the line and torch. I believe the idea is to prevent loose floaty line with potential for entanglement. Neat trick which I hope to try out some day, although it may end in tears if I end up loosing the spool and torch!