Distantly, Optimus realized that if not for the gag, his pleasured, silent moans would likely be heard as embarrassing whimpers. He was already arching into every thrust as though it might be his last.

He wanted to overload; needed it, but at this point he wasn't sure if he could handle Megatron forcing him back down once more. It actually make him reluctant to give in to the rising tide of charge.

Silent other than the rising scream of his overheating engine, Optimus caught Megatron's optics with his own - and squeezed down with his valve as tightly as he could manage, crackling charge beginning to send him over the edge despite the fear. Even if he could not hold him in, distracting him long enough to keep from pulling out and feeding in that charge for just a few more moments would be enough.

Don't you dare stop him; don't make him plead--!