At MER, it seems that all the moving parts are slowing down except H-R. The boost of energy in the H-R component suggests that potential energy stored by the other components (which have now slowed down) is released into H-R. In addition, we know that there is an active component adding energy to H-R at this point (muscles, mainly shoulder, I am guessing). At impact, of course, the abrupt transfer of energy to the ball causes H-R to slow down drastically. It's probably not a coincidence that the graph showing the drop in energy looks like the path of the racquet head after impact!
What is surprising is the differential in the maximum energy levels - 140,000 for H-R vs 40,000 for Tr! Surely, the shoulder/arm does not produce that much, does it? The work done by the other components must have stored the excess energy as potential energy in the internal rotator muscles of the shoulder, I suppose. I won't even try to guess what the efficiency must be, it is already too complicated!
Fascinating, Sennoc! I eagerly await your explanation!