Football rule question, Henderson stepping up into the A gap...

You've all seen the play, numerous times most likely. I don't get it.

Yes you can motion one player... but to my understanding NOT forward, or toward the LOS. That's exactly what Henderson does.

Now I'm not saying it's illegal.. obviously it's not... BUT... there had to be a rule change, and if so how does it read? Why can't you now have a WR, or Slot, or any other position, line up 10yds behind the LOS and before the snap take off like they're shot out of cannon so they hit the LOS at top speed?