It is positioning on a mountain bike. You need to keep weight over back and front of bike; so you need to crouch down lower than you would on a road bike to keep the weight over the back wheel a bit; also off road the hill can be steeper so you still have to keep the weight over the front wheel more as well or you will start to wheelie; it is a balancing act so you can be totally static on the bike. Smooth, smooth pedal strokes when it is really loose.