How is this move a brilliant move?

Because the trade forces the Bishop on G4 that is a unexpected move that's why

it is brilliant b/c it either wins a queen or lets you promote a pawn to a queen you are also sacking a bishop and if they take it you continue the attack with e3 and eventually they sack material and you win the game

I agree that the move is brilliant because it sacrifices the bishop on g4, and after fxg4 e3!, and the d-pawn is pinned and there is too much pressure to the point of resignation from the threat of exd2+ winning, the queen, and pressure with the pinned pawn.

The pawn is pinned to the queen and the queen is pinned to the king and the rook and the pawn stares at the pawn.

Lots of Pressure

My first Queen sacrifice is taking the rook for a passed pawn and promote it to a queen. Then a backrank Checkmate

Cause if white pawn on f3 decides to take black bishop, u push black e4 pawn to e3. Then white pawn on d2 cannot take pawn on e3 cause it is pinned by the black bishop on b4 to the white king. Then on ur turn u can capture pawn on d2, checking king but after capturing queen will be face to face with rook but ur king got checked so king has to escape check by either moving or capturing checking pawn with white bishop. Then rook takes queen.

https://www.chess.com/analysis/game/pgn/ifZr5jage/review?move=0This is one of a few situations that might happen if u play smartly