My understanding is that the strap works in a couple of ways,
Firstly, it spreads pressure across the patellar tendon and relieves individual stress points.
Secondly, it lifts the patellar slightly again relieving pressure - as a subset of this it can help tracking.
Finally it reduces vibrations across the tendon (caused by running).
That the theory anyway! Personally I have no confidence that it will work but am getting desperate.