You should always be in a state of powering down

Here in STEEM, your control over your deposited Steem in the form of SP is very week.
If you want to be able to sell your Steem, while also enjoying the benefits of having as large SP as you afford to yourself, you should power down constantly.
Should you conclude you want to keep more of your Steem as SP, you can power it up immediately after it got powered down as Steem.
Powering up does not stop the process of powering down.
Only 2 things stop your powering down, an explicit click on the stop powering down option in your wallet followed by your active key, and an end of 13th week of power down process.