Originally Posted by slidewayes
So you build the p.s.i. and see where it go's
Yes... except you don't build it, you charge it with compressed air.
Comp test measures the engine's ability to generate pressure. Leakdown measures its ability to hold pressure. Both will tell you if your engine is shot if that is all you want to know, but leakdown is more useful for diagnosis, and offers a bit more precision in measuring how hosed it is.
With leakdown you will charge the cylinder to X psi and measure the pressure loss over Y time. If the damage is there you can often isolate where by finding the leak.
In hindsight to your point about doing it yourself, I probably should have just bought a basic leakdown testing setup and then taken it to the dealer if I confirmed I had an issue for warranty claim but I want to get my bike to healthy state ASAP. Since I just moved all my stuff is still packed in boxes and the garage is a mess so I wasn't really in DIY mode and was working with only the KTM tool kit outdoors.