Black checked White’s king by moving his bishop to a6, and White replied by interposing his bishop on d3. We know one way to handle a bishop on the same diagonal as its king in this setting: trade it for another piece. Thus Black plays RxB, White replies RxR, and now White’s rook is pinned: a suitable target. The next question is how to attack it. BxR would make the whole sequence a wash after White recaptures; Black needs to summon more force against the rook. He can throw another rook of his own at it with Rd8, to which White would reply Rd1—and the pinned rook still would be guarded as many times as it’s attacked. What Black really needs is a pawn on a file adjacent to the rook that can be used to go after it. The obvious candidate is the pawn on e6, but it’s two moves away from the pinned rook. What to do?

Answer: march the pawn forward. Black plays e6-e5, and there is nothing White can do to prevent Black from playing e5-e4 on the next move and then e4xR a move later. White can play g2-g3, threatening Black’s bishop, but then the bishop just retreats and the plight of the pinned rook is unchanged. The reason Black’s pawn is free to march forward is that the few pieces White could use to stop it are committed to their positions. White’s king can’t move because it is protecting the pinned rook, and anyway Black has d2 and e3 sealed off with his bishop. Nor can White’s knight or rook be maneuvered into better positions in time to stop the pawn’s progress.

This is what sometimes happens once a pin is imposed: it requires your opponent to freeze his pieces into positions that help protect the pinned piece; his immobility makes it easier for you to make offensive moves with your other pieces. Sometimes the time it takes to move your pawn will give the enemy a chance to rearrange his pieces; that is why it usually is best if the pawn makes threats on its way up the board that require time-consuming replies. But as this position shows, that isn’t always necessary.