On steep stuff like that It is all in the front brake and having the confidence in barely touching the rear. When you hear the rear tire slipping you are using too much rear brake and in turn having less control. Often times on really steep sustained stuff like that you can even let go of your rear brake entirely. Of course body positioning is also important and you don't want to be riding your rear wheel either. That's why Remy can do a stoppie down the entire slab.
