when you delegate your steem power / vests they are being used by the delegatee (the project you delegated your power to). Because currently the steem rewards (curation / author) are calculated over a rolling 7 day window this is why there is also a 7 day waiting period before you can again use those steem powers. Otherwise imagine the project you delegated to just voted for something, then you get back your steem power and you also vote for that same thing with your regained power ... can't work like that :)