Hello, answered your question, when you do Power Up of your STEEM, each time you vote, your vote will have more value and you will be able to receive STEEM Power. Doing Power Up you can do it immediately if your account has enough. You can only do Power Up of your STEEM, so if you do not have STEEM but you have STEEM Dollars (SBD), you will have to convert your SBD to STEEM first and then do Power Up.
Power Down is when you decide to pass STEEM Power to STEEM to trade or sell them or buy STEEM Dollars (SBD). This will take 3 months and each week you will receive a quantity of STEEM, and this will continue until after 3 months.
You can cancel the Power Down when you want.
You must be careful because this may affect the reputation of your account.