Orion might not have it in him to swing the Forge extremely hard, but it's a very heavy hammer, and getting attacked by his prisoner is the last thing Megatron is anticipating – it crashes into the back of his helm with a ringing clang just as Rodimus seizes the Matrix, and he staggers forward with a snarl of rage, optics flickering. He clutches his head as he tries to regain his bearings – in a moment or two he'll be rounding on Orion (or the space where Orion was, if he's had the good sense to move), full of fire and fury, but that move has bought the Autobots a valuable sliver of time.