STEEM power is locked away, if you power down it takes 13 weeks to get the full amount out. So say if you have 130 STEEM power and start a power down, first week you get 10 Steem, 2nd week you get 10 STEEM etc. It is a slow release to prevent a market crash if everyone withdrew their STEEM at once and sold it on the exchanges. It also acts as a security feature in case someone has your keys and tries to steal all your funds. At least you would have time to change your keys when you notice a powerdown or a portion of your wallet is stolen.