I just had my recall accomplished. What they do is to run the motor at 6000 rpm for 20 minutes until it reaches a certain temperature. (My instrument display was set to show rpm and motor temperature when the bike was returned to me.) If the magnets don't fly off the rotor and lock the motor up, then the motor is deemed good, you get to keep your motor and you can ride off into the sunset. I was told that the problem was that the magnet adhesive was not applied correctly to some motors when they were assembled at the factory and this "stress test" is the method of discovering which motors might be defective. Fortunately, my motor passed the test.
One of the service guys said that a local cop had his Zero lock up while riding it at top speed. Other than that instance, they had not heard of any other failures.
The 20-minute stress test only required 5% of my battery power to complete. My S model made a high-pitch, but not very loud, whining noise during the test. However I was told that the HP SR model is really loud and can be heard all over the shop while it is being tested. I was also told that the service department only wanted to perform one Zero stress test per day due to the irritating (to them) noise that the motor makes during the test.