Thanks to the Rook Gambit, the rook isn't on a8, offering this very beneficial tactic. With tempo, as the knight is being attacked.

This threatens Qf2++. Note that there is no white square bishop due to the earlier Rook Gambit exchange, thereby contributing further to black's position.

This is the only way to stop mate.

A rook down, black bravely no longer cares about even such trifles as the solidity of his king's castle.

If Qg4+, Kh8, then soon Rf8 is to follow. The pawn move temporarily obstructs the bishop's protection.

The pawn isn't safe.

Still plotting Rf8. But also possible is Bh4, and if Qxh4 then Qg7++. And if queen moves, then bishop wins the rook.

With two imminent threats, it's a good idea to move the king away from the danger zone.

Still can't take in this situation because of Qg2++.

Gets tempo and saves the rook for white.