We've referred to this type of drill before as "short release", so we're revisiting this concept.
Why do it:
There are a few reasons to practice this technique, one would be to learn how to vary your stroke to avoid injury, another would be to shift all the focus of your stroke to the front. Additionally, some people find it helps them develop a more efficient stroke, while others see this as a way to increase stroke rate quickly during a race.
How to do it:
1) Swim normally focusing your attention on pushing all the way through the end of the stroke, even flipping your hand out at the finish.
2) Swim the next length lifting your hand out early, as early as the top of your hip.
3) Put on some paddles to give some extra push, and focus on the complete push through the stroke.
4) With the paddles still on, take the hand out early.
How to do it really well (the fine points):
Swim some 50's long course counting your strokes and setting up your rhythm. Using the same rhythm, see how close you can get the stroke count using the short stroke vs. the long stroke. You may be surprised just how close the two are, and how spending time focusing on this short release pushes your focus to the catch out front. Swim normally after these 50's and you'll see how much better your catch is.