I’m not that good at Lua either, I just started. But I will try to help.
I think you can do a local script and store the value of the player’s health in a variable. Then in a forever loop, check if the player’s health is lower than the last saved health (then do knockback). else set the variable to the current player health.
But I think it might be better to tell the player somehow, maybe through remote events whenever something damages the player (through the damage script)