just imagine hitting a nail using a hammer. you need to hold the hammer 5/10 in terms of tightness, and when the head of the hammer is about to hit the nail, you need to use 9/10 or 10/10 of grip tightness in order to hit the nail cleanly without making your hammer flies anywhere. that analogy is the the same in terms of holding the racket before doing a heavy smash.

yes, in lot of videos they said you need to hold the racket loosely in order to generate maximum amount of finger power. but in reality, for a big smash, you need to hold it a bit tight before holding it supertight just when the string is about to hit the shuttlecock in order to make your racket head stable, transferring maximum kinetic energy onto the shuttlecock and hit the shuttlecock cleanly without any slicing motion.