I popped back to the Wallace to have another look at the plates from Thibault that Dave R was demonstrating here. (and just spent 10 minutes explaining it all to an American actor)
They key point of the plates for me is that Thibault’s turn is very much on the spot, but crucially is only done when Alexander’s body is well past Zacahry’s point: he turns inside the blade. It’s a very restrained and careful action (apart from the context of ramming the rapier through someone up to the hilt) and there is no opening for Zachary to exploit.