The current algorithm looks at all posts and gives each one their fair share as if every post were to get paid out right now. i.e. If your post has 1% of rshares2, you get 1% of the reward fund.

The new algorithm looks at past posts and pays out based on previous claims to the reward fund. i.e. If your post has 1 million rshares on it and a post was paid out an hour ago (that also had 1 million rshares) got paid 10 STEEM, you could expect to be paid around 10 STEEM.