Fury is lucky ref intervened (IMO when he shouldn't have) and saved him from getting knocked out cold. Usyk was one punch away from puting him into sleep or worse.
Looks like a hail Mary but it isn't. He hit Fury several times with same punch in the previous rounds as well. It was Fury's punch resistance starting to wear down from the accumulation. It was a fun fight, Fury was eating many shots even when winning rounds and vice versa. Usyk's left hand to the body was the most consistent punch in the entire fight and it took it's toll on Fury, it set up the left upstairs.