Lesson
White to move. Main line: 1. d5 a6 2. dxc6 axb5
The bishop on b5 pins the knight against the king — the knight is legally frozen. d5 attacks it with a pawn, and since the knight cannot move, it is lost. Black wins the bishop back, but a pawn has traded itself for a knight.
The rule: a pinned piece is not really a defender, and attacking it costs you nothing. Count attacks against pinned pieces with the cheapest units you have — usually pawns.
The pin here is absolute — the knight is pinned against the king and may not legally move at all. A relative pin, where the piece behind is merely valuable rather than the king, can be broken if the defender decides the cost is worth paying, so count that cost before you rely on it.