The main aim of it is to do a normal Bunnyhop but instead of simply lifting both wheels at the same time, lift the front end up first and do a small manual, only when the front is high do you use your back foot to lift the back end up. This should result in a much higher Bunnyhop than normal. These 'American Style' ones can be handy for jumping up curbs at high speed without denting your back wheel or just for sailing up small walls.

Tip: when your learning don't go straight to the nearest curb you can find... make sure you know how to do it properly over a line or something first, ahem...