how to keep the script to check the player's position?

the player defaults Y is "6139.23". When their Y decreases, his Humanoid.BodyHeightScale will decreasing until 0.

first, how does the script check if the default Y is not shrinking, and how does it start shrinking then how the player starts getting lower?

secondly, where and what script should I give so that it works only on 1 player (i.e. if Y will lower for one player, ONLY THIS PERSON will start to shrink)?

Greetings, KotyOceloty