If you're doing a 40 roll, stay at 40 push down on the brakes, and give it gas. To use brake boosting the way its supposed to be used, you should have your foot all the way down on the gas. And still on the brakes, while still being at 40 mph.

Its not hard to do at all, everyone who said that your brakes take a beaten are full of it. Yeah they do, but your brakes won't go out like that, if you do it alot. I have 26k miles in my car and have been doing it since I had 12k miles and my brakes are still really good. The best way to learn is to have someone show you first hand.