Whose arm is it? Surely not Indy's.
Yes, it surely was.
Aside from the "kneels before God" turning into a roll, that snippet of the scene was completely plausible.
At the end of the [kneel] roll, Indy's directly facing the rope -- which is hanging on the wall right next to the wooden gear, and with a noose already formed -- and he analyzes it for a split second (Harrison Ford makes an upward gesture with his head to emphasize that he's considering the rope). Then, he takes the rope and stops the gear. You can also see the diary in his hand when he's attaching the rope.
IMO, a near instantaneous connection between the moving metal blades and the moving wooden gear could have easily been made. With Indiana Jones not only being an intelligent person, but also highly experienced with thinking on his feet, performing this feat is well within his wheelhouse of capabilities.