Originally posted on 08/21/2013:
That's a good question. All the instructional video's emphasize that that the hitter see what he hits. So, I guess simply hitting what you're not looking at isn't "illegal" per se, but it's damning if the hit is questionable. It's also dangerous for the hitter to hit like this, as it causes insane pressure on the neck.
Not sure the NFL knows how to define it, except "they know it when they see it". (Famous quote from a porn case)