If you want FAST turns and some interesting training, one good trick is to take the wall out of the equation.

Why do it:
Without the target of the wall, you can focus solely on getting your body into the tighest tuck possible, for the fastest flip.

How to do it:
1) Start by swimming freestyle, and simply doing a flip in the middle of the pool. This is just to build an awareness of turning without the wall.
2) Now, focus on staying in the tighest tuck possible, and with that tuck, allow your body to spin further than normal, all the way around. The goal for the swimmers will be to do 3-4 of these turns in a 25 swim. This will help you get focused on staying very tight as you have to spin further than typical.
3) Next, repeat the previous 25, but this time take it all the way to the wall for a regular flip... at the same speed and intensity.

How to do it really well (the fine points):
To really learn great turns, you need some great momentum. For this drill, use the wall. Push off and sprint to a pre-determined spot. When you get to that spot, flip quickly and get back to the wall. The extra speed can help simulate a more race pace situation.

If you REALLY want to feel some speed, do this same dril from the blocks. You'll quickly see that even though this is a turn drill, getting your momentum started again takes some good leg effort.