No, this doesn't make the number of available HITs multiply on their own. ;-) But it does make it easier to see how many HITs you have left in a batch, by adding a copy of the current "HITs Available: x" number below the HIT work area, so you don't have to scroll back up to look at it. Particularly helpful when using mmmturkeybacon Scroll To Workspace
Important point to add: For this to be useful, you need to also install MTurk Max Job Window Height
, which prevents external HIT content iframes from being taller than the available height of your monitor (so you can see the bottom buttons area all the time). I've been using it since nearly when I started turking, and forgot how oversized the iframes tend to be otherwise.
See also: my MTG thread post
For many more MTurk-related userscripts, see the Turking Scripts set