Knox sprinted down to that corner from his position up on the line against Bush. If you watch the original angle that was from behind Masthay, you can see him release and sprint down the sideline. You can also see the hold of Bush. It comes into the screen on the left just as Masthay gets the punt away. From the still shot sequence on JSO, it looks like #21 actually hooked the collar of Bush's shoulder pads from behind and pulled his shoulder backward. Bush's upper body twists backward even though his is past the blocker. It was probably pretty evident to the official.