The system is comprised of four pieces: a custom bulkhead (to suit a range of different housings), a one-way waterproof valve, a vacuum gauge and a hand pump. Together they form a system that can test the integrity of your waterproof housing. Once everything is in place, you use the hand pump to create negative pressure inside the housing, check the gauge, and then check the gauge again to see if the pressure has held. If you are not in any sort of a hurry you can run the test overnight. It offers a lot more precision than the old-fashioned method of dumping your pricey DSLR rig into a bucket of fresh water and looking for tiny bubbles.
Waterproof DSLR housings are never cheap, so it should come as no surprise that the Backscatter AirLock Vacuum System has a hefty price tag all of its own. I am guessing that the first two components (the bulkhead and the valve) are responsible for most of that – it is certainly not an area in which to cut corners.
Considering the fact that a good underwater housing often costs as much (or more) than an actual camera, the $400 price tag seems reasonable.