I'm not sure a video would show it, unless you were filming at exactly the right angle behind the TSA agent.

However, in case I'm wrong, I'm sure there are others (perhaps even yourself?) who would be happy to volunteer to prove otherwise. Especially those who have no problem with having videos of other opt-outs taken and posted online, but who haven't yet had a chance to be a part of this themselves, perhaps they could participate with their friends at their home airports in opting out and filming and posting online as I described before. Given the variability of TSA agents and in how they interpret the policies, especially if this were done at many different airports, it's likely that at least one or two cases of karate chops, and maybe more, would be uncovered.