We have been saying this for almost twenty years now. Proceed with caution!
If you are trying to imitate the pro model, which you should if at all possible and of course safe, beware of mainstream instruction. That includes on court and online.
There is plenty of slow motion video and stop action photos that reveal the exertions, the order of exertions and bio-mechanics of the pros. You may get the ah ha moment yourself just from viewing or you may need some videos and pictures of your own game and then compare.
Assume NO ONE knows what they are talking about! We have seen many years and many dollars wasted. Do NOT let anyone explain or try to tell you what they think the pros are doing. Or worse, insist on their “system”.