Why this is a brilliant move?

Here is a position played by someone rated more than 2200 ,White just played a brilliant move according to the engine.

I just cannot understand,if Black just take the rook ,the position will be 

Engine says White is better.But why? Is it because White has a d6 passed pawn?

Here is the link to the  entire game

https://www.chess.com/analysis/game/daily/412978797

https://support.chess.com/article/2965-how-are-moves-classified-what-is-a-blunder-or-brilliant-and-etc

Under the definition, it just has to be a decent sacrifice.

Brilliant (!!) moves and Great Moves are always the best or nearly best move in the position, but are also special in some way. We replaced the old Brilliant algorithm with a simpler definition: a Brilliant move is when you find a good piece sacrifice. There are some other conditions, like you should not be in a bad position after a Brilliant move and you should not be completely winning even if you had not found the move. Also, we are more generous in defining a piece sacrifice for newer players, compared with those who are higher rated. 

I think that if black takes the bishop on a8, white's pawn on d6 is blockaded. The position is extremely complex.

The pawn on d6 can possibly be protected by the bishop on d2.

Thank you!

But my quesetion is not how chess.com define what's a best move,but how the player can find that move(Bg4) ,and why after some trades of pieces(like the real game or just after Black Rook take b7), white is better.