Here's how Miami lined up in the Wildcat on Brown's long run. What I noticed was that the formation is not your typical unbalanced line. And at the snap the right side of the line ran what amounted to a trap block: the center engaged the guy over right guard, and the right guard blocked into the hole. They did still have the left guard pull into that hole. But the defense was obviously not prepared for such a move.
They normally would have expected the unbalanced line with the typical blocking schemes. Instead, the Dolphins were able to get the line blocked and take out the linebacker. Since it was a quick execution, Brown was behid Smiley (the pulling guard) as soon as Smiley got there.
The gain was a long one.