Yeah, it's only on Synthesis because I had to do something and so didn't get a chance to add it to other styles.

It works like this:
On your own posts, you can see a bar that indicates your activity for the day. When you have filled the bar, further posting will do nothing to increase your overall activity. The red star is the points for visiting, the yellow stars are points for posting. Mouseovering reveals these details.

The reason for this is several posts of "I posted a million posts today and my bar hasn't gone up". There is a simple reason for this; the activity bar itself is updated every midnight CST. This removes flutuation due to removal of points from the day 28 days ago only to be added again at random times on the current day. The new bar makes it obvious when further posting will have no further effect on your activity bar.

It's only above your activity in your posts because I know most of you would just spam a few extra times before you would go to the trouble to go the forum index or your CP to find this info.