The frame flex unflexes on a dead part of the pedal-stroke when you're not exerting as much force. This typically is around the bottom of the stroke before the downstroke on the other side. The unflexing opposes the downstroke-force on the other side and gives you a little more force to push the pedal against. It's kinda like the end of the wrist-flick in squash.

Titanium is very elastic compared to steel, closer to CF. It springs back smartly and returns all the energy. If it didn't, the frame would get hotter and hotter as you ride and melt.