See my other comment here: https://news.ycombinator.com/item?id=16468635

I can feel the backbeat pretty strongly in other music, but this clip sounds very 1&3 to me. It would be musically foolish to automatically prescribe 2&4 to every blues and jazz tune as an axiom. At that point, it just becomes a shibboleth.

Frankly, I think they're just clapping twice as fast in that clip. 2&4 sounds weird and off-rhythm at that speed (to me), but totally normal with half the claps.

(Also: now I'm going to be self-conscious about my clapping until the end of time. Thanks a lot!)

To me, 1 and 3 is just a little off kilter. It doesn't sound quite right.