You receive VESTS (see how many of those you have at steemd.com). There's a pool of Steem backing the VESTS, and here on steemit you see the corresponding amount. The pool grows, so your amount of Steem grows.
For the experiment: during the first 30 minutes, a part of your rewards goes to the author. This was introduced because bots were always the first. When you vote after 15 minutes now, you give 50% of your rewards to the author. 33% after 20 minutes, and so on.
So it's better to vote after 30 minutes?
Depends - when a lot of people vote at 28 minutes it's better to get in before. It's a little game on its own ;)