You can power UP and your committed for 2 years but your not subject to the STEEM dilution. You can Power Down and you'll get a withdraw every 7 days based on when you hit the button (or you can cancel at any point). The withdraws are equal but will you will see a bit more STEEM each time to compensate for the extra STEEM in print.
The Web UI does a full power up and power down (all Steem or all Vests respectively) although the blockchain supports the power up or down of any amount.
How is it not subject to dilution?
For every 1 steem created to pay others 9 steem is created and distributed to those with steem power.
9 total steem across all users? Is there any proportionality or is it all the same to everyone?
Awesome response, thanks for explaining it.