It's a safety feature to prevent engaging an ATX into drive at high RPMs. I'm pretty sure dropping it into drive at 4K would probably grenade the tranny anyway though... This is just enough RPMs to check out that the secondaries are opening.
This safety feature basically engages the rev limiter which stops the spark until the revs drop to a safe level. When driving, if you shift 1 to 2 manually you will notice the engine will stutter if you let the revs climb above 6750. (Not that you should shift like this, but hey, it could happen.)
