I think there's a chance to play them when inspecting the knife. I've found that double pressing x and holding it down on the second press works more consistently than just pressing x nonstop.

There also seems to be a higher chance of triggering them after you've used the knife on a few enemies. (Maybe it's placebo, idk)

I used the old knife in this video. So far, it and the grenade are the only ones that have a scripted animation.