I had the great pleasure of watching a pro do this last week at Miller, and here is what I have to say about it.
As we entered turn 1 on the east course it was clear that he was using the rear brake to lock up the rear wheel. As he did this he countersteered to pivot the bike onto the line he wanted to be on, sliding the rear wheel. Then he hit the gas, and it was like he was shot from a cannon. It was really amazing.
I'm a long long ways from being anywhere near that level. It seems that it is a very effective way of going around a corner.