It is tested, but...
The company has apparently done a number of tests to show that an installation with this device can achieve the same requirements specified in federal standards. The catch is that the federal standards themselves do not regulate any aftermarket products in any way. In addition, carseat and vehicle manufacturers also do not recommend such products. Basically, if you use any type of aftermarket product you do so at your own risk, knowing that it is not itself regulated by any standard.
Does it work? In some cases, probably yes, in other cases, probably no. We generally do not recommend any such products. The conservative and safe alternative to aftermarket products is to first see a certified, local technician for a free inspection. The second is to purchase a different restraint that will better fit your child and/or vehicle without the need for aftermarket devices.
Obviously, if you have exhausted all the options you may have to make a parental choice of your own to use a product that does appears to be the only method for a good fit. It certainly is possible that such a device may be the only way to get a good installation in the rare cases where a particular carseat wont otherwise fit into a specific vehicle. Please see a technician first, though, you can locate one with the "Free Carseat Inspection" link under the top banner.
Darren