I only found a few posts on this, but its basically a hydralic dump valve that allows you to bi-pass the proportioning valve to give the rear brakes most of the line pressure and lock up the rear wheels at the line without shocking the drivetrain like a trans-brake. They've been around for a long time. There was an article about 2 years ago in GMHTP, and I found a post that said Kirbans had them but can't find them on their website...