White's rook on h3 is threatened with capture by Black's g-pawn. But White has the makings of a threat of his own: his pawn on e5 masks his bishop. White’s bishop is aimed toward the Black king's territory, the king’s pawn cover is blown, and the e5 pawn can attack Black's queen with a step forward. That move—e5-e6—threatens mate (Rh8), but again the drawback of a mate threat is that your opponent has a wider choice of replies than he does when you give check. Here he would look for a move that defuses the threat and moves the queen out of danger—and would find it with the simple Qxe6, devouring the pawn and giving his king a flight square on f7. No, the better course is to first draw the king into position to be checked by playing Rh8+. Black must play KxR or Kg7; in either case the king is moved onto the long diagonal. Now comes e5-e6+. Black plays Qg7, offering to exchange his queen for White’s bishop to avoid mate. You accept the offer.