I'm not sure at which steem power it unlocks. I remember many months ago they moved the feature down to like 500 steem power. I was higher than that at the time and I suddenly had the slider. I don't know if it is lower than that or not.
It gives you a little slider from 0 to 100%. It remembers your choice for posts and comments. For example I tend to upvote posts 100%, and comments 10% but, you can slide that slider around on a case by case basis if you want.
When you click the up vote arrow a slider pops up to let you set how much you wish to use:
Thanks for the detailed explanation, just 400~ SP to go :P
Unless they lowered it. You'll know it when you reach it. Hard to miss.