Black experiments with the effects of advancing a pawn and finds that e4-e3 moves near to forking White’s queen and rook. Of course White wouldn't allow that; but go farther and ask precisely what he could do to prevent it.

(a) White won't be playing d2xe3 because the d2 pawn is pinned to his queen. (In other words, Black would have QxQ; if White then replies RxQ, Black plays RxR#.)

(b) If White responds to e4-e3 by moving his queen out of the way (say, to c2), Black plays QxR#.

(c) If White moves his rook to g1, Black can't quite play BxR, because his own pawn would be blocking the way on e3. But he can use that pawn to take White's pawn on d2, and thus threaten to take the c1 bishop next move; once White avoids that threat (with Bb2), then Black has BxR after all, winning the exchange and a pawn after White recaptures.

(d) So suppose White instead plays Re1. This, too, fails—to e3xd2, forking rook and bishop. White plays Bxd2, and then Black has QxB, winning a piece.

The important thing to notice is how a pawn can march not only forward but also diagonally by making a capture, expanding its potential to inflict forks. And then there is a larger point to observe: the pressure on White’s king that indirectly drives the tactical sequence here. The king is stuck in the corner; the Black queen’s threat to mate on White’s back rank (QxR) effectively freezes White’s queen in place, as it must defend against this possibility. These pressures on White’s king do not enable Black to mate, but they do constrain his other pieces severely enough to make a capture of material possible. It is important to appreciate how such accumulations of pressure against one point—especially though not only against the king—can end up paying off with gains elsewhere as your opponent has to make sacrifices for safety’s sake.