Typically you are allowed one defence of your line.

After that your are blocking which is a capitol offense.

After watching the slow mo, its hard to tell as the angle comes from the side of the track and i cant see if he was checking his mirrors.

Before the slow mo though before they crest the hill, he has already made his way to the inside so the move during the slow mo was the secong defense.

That's the one that is not allowed.