how do i copy animations from one entity to another?

i want to make something that stays on your arm like a wristwatch and i'm doing it with an entity and a plugin that adds entity models to the player model because i can't do it the way i'm thinking as an item, but the problem is that I don't know how to make the entity synchronize with the player's arm animations, I know it's easy to make it follow the arm swing animation but I'm having a problem with the animation of his arms "shaking" while he is stopped