basically in order to stand up while you are trying to controll the bike, and accelerate hard requires you to be one with the bike, meening holding on with your legs instead of your arms. Once you are holding on with your legs (predominantly) then you can begin to practice shifting your weight by pivoting at the waste, and the knees, wrather than at the pegs. Thats not to say that your feet should always be still, but you can keep much better control if you can shift your weight without changing your whole hold on the bike. Plus, if you can't then that meens whenever you move around on the bike, your arms have to take all of the load before you can get back ahold of the bike with your legs.