It don't mean nothing! Just like a chicken when you chop their heads off, they still flop around for a while. Its just a reflex of the nervous system. The sudden shock to the spinal cord sends out an impulse to the muscle for them to react, but there is no control after the impulse is sent due to obvious reasons. For birds that just drop, the hit was so quick that there was no time for a reaction of the nervous system. Everything is just shut down instantly.

BM