The pull-up mainly uses your biceps and your lats, which are exclusively upper body muscles. So would raising your legs during a pull-up reduces the energy used in those muscles? Let's say the energy used in your abs and cores are irrelevant here, even though in reality your lats do somewhat stabilize your core.
Arguments for reducing: When you raised your legs, your center of gravity moves a shorter distance, thus you use less energy to hoist your body up.
Arguments for not reducing: Because your upper body muscles move the same distance regardless of whether or not your legs are raised, and because they hoist the same weight, they use the same amount of energy.
So which one is correct?