CharacterUseJumpPower determines the starting value of Humanoid/UseJumpPower for a Player's Player/Character|Character. Toggling it will change which property is visible in the properties window: StarterPlayer/CharacterJumpHeight|CharacterJumpHeight (false) or StarterPlayer/CharacterJumpPower (true). Defaults to true.

Since this property is only relevant for characters being spawned in the future, changing it will not change any existing player characters. Changes to this property will only take effect when a player respawns.