It allows you to adjust the amount of air that bypasses the TPS. It allows you to allow more air past the closed TB during idle. So if your car is surging at idle, sometimes you can allow more air to bypass the TB to calm it down.
However, they should really call it a band-aid, because it doesn't fix anything. All it does is cover up the problem that is causing the surging idle in the first place. It is no different than just upping the idle screw.
You guys with idle problems could same a lot of time chasing your tails if you would go through the Surging Idle Checklist. Over 50 different people contributed information to it. The first post has all the fixes, and steps through the how to find and fix you idle problems without spending a lot of time and money. I continue to update it as more people post fixes or ask questions. You can post questions to that sticky and have your name and idle problem recognized. The guys with original problems and fixes get their posts added to the main fix.
I bought one of those things, didn't do much of anything. Except remind me that the problem was really that I need to get a PMS tuner to get the thing to idle right. Usually its just a vacuum leak though that causes surging.