[nesnah]

That’s a very odd clenching. I tried to replicate just now. Very unnatural.
.

[TornadoAlley3]

More natural to put thumb inside 4 fingers.

[DaxtonBrown]

“That’s a very odd clenching. I tried to replicate just now. Very unnatural.”

I had a broken neck when young and have some paralysis in my one hand. That sure looks like some of the non-voluntary spasms I sometimes get.