The sente drops Bishop at at 4c.
Note you can also drop Bisop at 4a. The result will be the same
Bishop takes Gold. +4 points
Rook takes a Bishop. -6 points.
Pawn promotes and immediately threats the Rook. promotion is +6 points. So total score is +4 points. Even sacrificing Bishop, the attack that will create a promoted pawn tend to be a good exchange.