you have to learn to modulate the brakes enough to change your position while leaned over. Additionally, you have to learn to use counter-steering to adjust your lateral position while leaned over. These techniques will allow you to make subtle changes to your position while in close quarters and respond to immediate situational changes. If someone has the high ground on you (meaning they are already in front of you) then if they move into you, you have little choice but to change your position to stay safe. I suppose you could try pushing off them elbow-to-hip, but that seems iffy at best, especially if they have more mass.