The half pipe would be the best analogy, and to get the maximum energy transfer to speed up you need to be short on the way down and tall on the way up. To slow down you need to be tall on the way down and short on the way up. The center of mass needs to change at the moment when the largest component of the centrifugal force is aligned with gravity (i.e. bottom). The change in energy will depend on how much the center of mass can change, how close to the bottom the change happens, and how much angular momentum there is. If there isn't any angular momentum then there will be no effect. The centrifugal force must be maintained the entire time the center of mass is changing to get the full effect. So, in Chuck Martin's example where he "punches the bump", he hasn't had any time to generate centrifugal force in line with gravity, and at that moment of pressure there's very little change in the center of mass. Most of the compression happens after he releases the pressure therefore there's no more energy transfer while he's compressing. There are definitely times where bumps are shaped with enough curve to get effects from pumping, but one thing I think about is that speed control with A&E (for typical technique) is not so heavily dependent on the lip of the trough. If a large part of speed control was from this effect then if the curvature of the bump was largely changed then speed control would fall apart. But, we don't see that. Any of these effects that rely on the up hill part of the lip of the trough to work, can't be that important, because we don't see sudden changes in speed control efforts as the size and shape of this lip changes by factors of 2 or 3 or much more. Therefore, the best skiers at fast speeds actively pull their skis up to avoid their COM moving higher to avoid additional acceleration from gravity, the dominant effect. Somebody pumping would need to have full force the entire time they are compressing, which also ends up pushing up the COM, so there are competing effects, and with the large variations in bump shape, technique is largely based on avoiding acceleration down steep slopes.