Start with the convergence of Black’s queen and bishop on f2—the makings of mate, except that White’s queen on c2 guards the square. Black has no good way to get rid of White’s queen, but on inspection of the line connecting it to f2 he hits on another idea: drop a piece onto the second rank and obstruct the White queen’s path. Black can do this with his rook or bishop, either of which can go to e2 with protection from the other. Re2 is no good because is allows White to extinguish the mate threat with QxQ and then to win material rather than lose it. Be2+ is crucially different because it gives check and so keeps tight control over the initiative; White has a few ways to reply, none of them appealing:
(a) He can take Black's bishop with his knight or bishop; this still leaves the second rank blocked, enabling Black to mate with Qxf2 next move.
(b) White can play 1. Kg2. Then it goes 1. …Qxf2+, 2. Kh3, Qf1+; 3. Kh2, QxN+; 4. Kh3, Qh1#. Black’s queen and bishops are too much for White’s king to outrun.
(c) White can take the bishop with his queen, reestablishing its protection of f2. Now Black plays RxQ and White dares not recapture, as the guardian of f2 is off the board (so BxR leads to Qxf2#). White is better off replying to RxQ with Rf4, which blocks Black’s mating attack for the moment.