Figure[White to move]

Lest chasing pieces with pawns seem too easy, consider a limitation on the idea. Look for potential captures White can make and you find QxR. The good news is that the rook is protected only by a fellow piece, the knight on b5. The knight can’t be taken, but White has a pawn to throw at it: a3-a4. It might seem that the knight must now flee, and that here as in the previous position Black is left with a hanging piece for White to take. Not so. We encounter again a fundamental limitation on the use of attacks (like a3-a4) to force results: in itself the move doesn’t cost Black anything; it merely threatens him with a cost. That means he still has time to threaten countercosts of his own. In this case Black looks for a threat that he will be able to execute if White goes through with a3xN. He finds Nd7. White’s queen must retreat, which gives Black time to likewise retreat his b5 knight—and without worrying that this leaves his rook unsafe, since the rook’s attacker has fled.