So I can't find any decent video of him from the front view that isn't years old, so it is a little hard to tell certain things. And clearly there is no high speed video publicly available either, which is really the only way to truly analyze mechanics. But I think what you are referring to with the "snap" at the end is really a deceleration thing.
Pitching mechanics really come in 2 phases - acceleration and deceleration. Accel injuries are typified by the soft tissue that is facing the plate during the delivery decel by those facing away. So once the ball is released, you have to slow an arm moving 95 mph at the fingertips down in a very very short time (just think of how long it would take to stop your car going 95 once you slammed on the breaks for comparison). The breaks on your body are the larger muscles on the back and in the shoulder (lats,superspinatus, teres minor/major, etc.). But one of the critical factors in slowing it down is shoulder rotation itself because that allows your arm to clear your body.
McKenzie doesn't seem to have real good shoulder rotation, so his arm seems to hit his body and therefore recoil back towards home plate. That is the snap I think you are seeing. His arm bouncing back towards home plate where it was traveling towards second an instant before. Think of your car being rubber like a hand ball and bouncing backwards after a 95mph encounter with a tree or wall. That is kind of what their arms do. Watch his arm closely and you will see it hit his back and then the next instant be headed back to the plate
Guys that don't rotate their shoulders well after the ball is delivered have their arm "recoil" and put a lot of strain on their shoulder. The bicep area of the humerus hits the peck area of the chest and comes to a dead stop while the rest of the arm is still zipping along. That creates a fulcrum were the lower arm literally "tries to pry" the rounded head of the humerus out of the shoulder socket. Lots of rotator cuff and labrum issues.
Think Danny Salazar in some ways as he didn't decel real well, even though he had other issues as well. So I am not saying this or that, lest I be admonished and keep the narrative rolling that I am "negative" about all other Indians pitchers except a past one, because I don't have the correct film angle or slow motion to really tell. However, my trained eye says that he will have shoulder problems if he has problems, unless a change in decel mechanics is something that he focuses on.
Probably doesn't answer your question real satisfactorily, but without a dissertation, which this is already approaching, and a whole bunch of video .... this is the best you get from me at least.