Black "copied" the opponent's bishop manoeuvre, and with good reason. Now the a2-pawn is in danger.

With a decisive grip on the queenside.

Preparing a "brave king" manoeuvre.

Thus White covers the sensitive f1-square and threatens mate himself.

The king escapes and White wins.

Correctly estimating that White's attack can be parried by a "brave king" manoeuvre.

Other moves permit Black to consolidate by 5...Re5!.

. Preparing 7...Kd8. By now the king is out of danger, whereas White has problems on the back rank. Black went on to win after

followed by ...d5-d4.