/me sighs
Lets see if I can shed some more light on the subject.
A PokeBody is always on. There is no choice to use it, well... there may be a few exceptions, but otherwise it is always on. Considering you have said you know how PokeBodies work, I will not go into them anymore.
A PokePower on the other hand, you CHOSE to use. If it says Once during your turn, then you may use it once during that specific turn. Once your next turn rolls around you may use it again. If it says Once during your turn, then later on, You may only use one "Name of Power Here" PokePower per turn, then you can only use that specific Power ONCE that turn. For instence, if you have 2 Pidgeot FRLG out, with the PokePower Quick Search, you may only use one. There are some Powers that say: As many times as you like during your turn you may use this Power. An example of this is Electivire from DP, you can move as many Lightning Energy as you like from your other Pokemon to Electivire. Every turn the "counter" of sorts for your Powers resets, so if you used a Power one turn, your next turn you may use it again.
Hope that helps.