Look at what happens when you remove the defender. Take the B-file pawn away from the board for a moment and look at how open that white bishop on the c-file becomes.
By removing the b7 pawn, it would leave the bishop open. Thus your bishop would be safe. The puzzle is assuming that black takes your white bishop and you take their white bishop thus getting a pawn and a bishop for a bishop.
Bxa6 - b7xa6, RxBc6
I am very new to chess, and decided to start doing some tactical problems. But, this following problem perplexes me...
Apparently the optimal move is Bxa6! (Bishop takes a6... good move). This "exploits the pin down the c-file to win a pawn".
But to me, this makes absolutly no sense. Wouldn't black's b6 pawn simply capture white's bishop... seems like a stupid trade off.