Way down at the bottom of the Cambridge dictionary page:
nail (verb): (informal) to do something successfully:
To nail something is an idiomatic expression that means to do it more or less perfectly. The guy in the video is saying that if you are not nailing perfect form each time, overuse (lifting too much or too often) will create problems.
The gymnast fumbled a little in the middle of her routine, but since she nailed the landing the judges gave her good scores.
I'm not sure about some of the questions on the exam, but I nailed the essay.
(edit) "Nails-perfect" is not typical of the way I've heard nail used as a verb, but language evolves and perhaps it's common slang in the speaker's circles.