Pretty much this. A couple of other points: I wonder if the mounting of the bike on a static trainer means the effect is over egged? The trainer effectively channels the force the only way out it knows, which is kinetic energy through the back wheel. In the real world, some of that energy could actually be absorbed through you alternate leg for example? As per later on in the thread, there will be some loss through kinetic energy in the sideways movement. Think of Newton's cradle. In my head I think where there may be an advantage in a flex frame is on rougher surfaces, where some dampening effect might be of benefit.