I always assumed half-wheeling meant overlapping the bike in front of you by just enough so that you were likelyto knock yourself down (as in "half a wheel") - and if the group is clumpy it's easy to happen. I've also heard, though, that this also refers, in a paceline, to being just a tad out in *front* of the rider next to you (and I assumed that meant at the front of the line, but now I'm not sure) - sort of saying "well, none of you are really going fast enough for me, nyah nyah."