There is a good view, the closest I've ever seen, of how Mosconi slipped at 12:55 of this Mosconi-Fats tape. To get to this point in the tape, move the slider with your mouse. Although pdcue may disagree, I would call it a small slip stroke. But whatever you call it, his grip hand moved back by two or three inches or so on the final backswing.
http://video.google.com/videoplay?docid=-3109100860459966589#
Oh no - the video evidence again - to avoid boring everyone with way too
much explaination, it apears you don't understand what a slip stroke is.
Willie also, in his instructional video, can be seen sliding his hand back
on the cue just before he pulls the trigger on a couple of shots.
To repeat, this is NOT a slipstroke.
If you have ever watched a slipstroker - you only need to glance at
virtually any shot.
The OP wanted to discuss the slipstroke.
HTH
Dale