So, this came up in another thread discussing death by guillotine in general, and I remembered a history teacher at uni telling us (as we discussed R.'s execution and the events that led up to it) that he was so impaired by the blood loss from his self-inflicted gunshot wound he couldn't or wouldn't stay on the table when it came time for his execution, so the executioner, after removing the bandage, flipped him over on his back and held his legs down until the deed was done- the result being that he watched the blade descend until it decapitated him. Is this...did he invent this story? Is this part of the eyewitness reports from the event? Try as I might, I can't find a single source to validate this. Was this just my teacher's way of dressing up what was already a pretty intense tale of R. getting his karmic due? Thanks!
Hmm ... I'll have another hunt around Gallica later, but contemporary depictions of the moment of his death suggest not. You can zoom WAY in and have a good look at how the guillotine was set up / how Robespierre's body is arranged.
Rayner & Stapley's The French Revolution : 1789-99 contains eyewitness accounts of Robespierre's beheading; none of them mention the details you've described.
I can't see any compelling reason why Robespierre would have had to have been flipped onto his back. If anything, that would have made his head more unstable in the cradle (the head is supposed to hang forward, creating a straight, exposed neck). If he was limp or weakened, there would have been no issue. If he had been struggling, holding his legs and his head / arms or just tying him down (pulling the neck into the cradle) makes a whole lot more sense than flipping him over.
As I said, I've never seen a primary source that said anything about being flipped over. Lots about his shattered jaw and him screaming when the executioner ripped off the bandage holding it together, but not the rest. Entirely possible that this is just post-revolutionary mythology.
If you want to read some crazy, documented analysis of Robespierre at the time of his death, the Lancet published some really cool findings based on analysis of his death mask late last year.