The first Genemutant with a third hand... *sing

Very nice trick

His wrists weren't very tight together when he was tied up.

Notice the cord wrapped around his knees, while the cord is tied, it isn't fixed with a knot. So even after his wrists are secured tightly, all he needs to do is twist his bound wrists to the side to release the pressure of the cord around his knees to make some slack in the cord. Notice that when is assistant is bent over him to cover his hands with the blue cloth, the assistant's body blocks some of the view too. At this time, the guy, dips his left elbow in for the twist.

Additional speculation: the guy might have had some sort of removable object inside his pants at his knee to act as a spacer that can be released as a precaution.