We're looking at ways to create pins where the alignments for them don't yet exist. We have seen how checks can achieve this by forcing an enemy piece to step in front of the king or by forcing the king itself to move. Now let's look at how attacks on an opponent's king can force him to make captures that likewise may result in pins.

A vulnerability in Black’s own position here should jump out at you: he has a loose piece on the same rank as his king; they are poised to be forked by White’s rook with Rc7+. But it's Black's turn to move, and before moving to stop the fork think about offense. Black has two checks to consider: Rd2+, losing the rook to NxR; and RxN+. In reply to the latter White would not be able to interpose anything, and if his king flees he has forfeited his knight for nothing. So the likely response for White would be a capture: KxR. Since this moves the king, reassess its position. It would be on the same diagonal as White’s rook. This calls for a pin using the light-squared bishop: Ba6.

Now what? It's White's turn to move and his rook has become a paralyzed target, so ask what defenders he might add to it. (Remember that White already sacrificed the exchange to get here, so he wants to win back more than the exchange now.) White could play b2-b3. But then Black has a pawn of his own to add to the attack with d6-d5. Black wins the rook. (Black also has another option: play BxR, inviting the recapture b3xB—and now Black’s a-pawn and b-pawn can be marched forward, and the a-pawn soon will promote.)

You might have noticed from the outset here that Black can impose a pin with Ba6—a relative pin of White’s rook to his knight. Since the knight currently is attacked once and protected once, it will be lost if White exposes it to capture a second time by moving the rook. But the pin fails because White can break out of it with check (Rc7+ or Rf4+). This illustrates why absolute pins are so powerful: if a piece is pinned to its king, it can't break out of the pin in this way.