What is a slip stroke and why is it used?
Slip stroke — The grip hand slips back on the butt just prior to the forward stroke at the cue ball.
Stroke slip — As the forward stroke at the cue ball is executed, the cue slips through the grip hand to some degree (a “throw” or “release and catch”).
Here is an article from Billiards Digest that describes several kinds of stroke including the slip stroke: http://www.sfbilliards.com/articles/2005.pdf (October)
And here is one that describes some other strokes: http://www.sfbilliards.com/articles/2005.pdf (November)