One of our current motifs involves asking what you now attack and playing with the answer. In this case the answer mostly is just the knight on e4; you pressure it with your rook. But the knight has protection and there is nothing on the other side of it. So now move to the second part of our searching technique: ask what you can threaten with your next move, and what would lie on the other side of it. Or look for enemy pieces on the same line. Either way you should see that by playing 1. Bf5 White attacks Black’s knight and that Black’s queen is on the other side of it. The queen protects the knight, but then the knight also is attacked already by White’s rook. So after 1. Bf5 it is attacked twice and defended once, and it can’t move. What further protection can Black add to the knight? Nothing. There is Re8, but it results in QxR#; or Black could move his other rook to b4, but this loses the exchange to NxR. The e4 knight therefore falls on White’s next move. The relative pin here functions like an absolute pin.
A large percentage of relative pins occur, as this one does, along diagonals, so looking for enemy pieces and pawns on the same diagonal is a good habit to cultivate.