"Writhing" (or maybe just convulsions that looked like writhing considering the restraints) after he no longer appeared to be conscious. NAD but my guess is this was a hypoxic seizure, an event of which he couldn't possibly have been aware.

Edit: Assuming I'm right, and if we ever get to the point where this is proven to be a "humane" form of execution, then the convulsions could reasonably be prevented with muscle relaxing drugs the same way they're are prevented during surgery or lethal injection... But I'd rather we just get to the point where we see the whole concept of retributive justice as inhumane.