Originally Posted by
Drakonchik
Like I said, equal and opposite reactions to the same bumps. You sink into the bump, or you diliberately rise above the bump. In either case you are jockeying/pogoing/moving out of your theoretical ideal distance (and orientation) to your cranks. (Or third case where you are riding the rigid, don't rise, and are therefore propelled.)
"Semi-standing" means using additional energy to change your orientation to the bike & cranks. Sinking into a hit utilizes learned reactions and anticipation -- not energy.
Methinks the skilled rider's body will notice and adjust to any propulsion up (or sag down) regardless of source in order to stay within a "fairly ideal" zone. ("Fairly ideal" rather than a fixed distance from sit bones to cranks.)
Suspension seat posts are most often not used by skilled riders, but unskilled riders on hybrids, and for them there is probably more loss of energy, with a suspension post, as they tend to put a lot of weight on the seat. Most of them who switch to a solid post notice a definite increase in pedalling power.