Staying near the Pocket
Both gaining speed with gravity and rail engagement have something in common: they generally are done near the pocket. This especially concerns surfers riding smaller surfboard.
Watch where experienced surfers ride on a wave on shortboards or fishes. They never go very far from the pocket. This is no coincidence. They have the experience to know where to generate speed on the wave using gravity and rail engagement, and this place is near the pocket.
Watch at 2:12