Black’s queen has a check that must be seen (queen checks always must at least be seen!): Qb4. When you examine a queen check you are looking in part for forking ideas; the question is whether Qb4 attacks anything else. Not directly, no, but from b4 the queen is aimed at the knight on h4, which is loose and therefore a target. The problem is the White pawn on f4. Can it be eliminated? Best would be to take something the f4 pawn protects, but that’s not going to work here; in reply to Nxe5, White has QxN+. So toy with more direct threats to see what they do. Thus g6-g5 is natural to consider, and then you see that it does more than threaten the White pawn. It attacks the knight, too, which has no safe place to flee—an occupational hazard of a knight placed on the edge of the board. So White is about to lose the knight unless he takes the Black pawn that threatens it; yet if he plays f4xg5 he loses the knight anyway to Black’s queen fork Qb4+. (White also can reply to Black’s initial pawn push with Qd4, preparing Qxa4. Or White can play 2. Nxf5, QxN. The material outcome is the same.)

There is a matter of move order to consider. Black could start with Qb4+, forcing Kc1. Then comes g6-g5, and again White must lose the knight (in effect White’s f-pawn is now pinned). Why not do it this way? Well, you could. But now if White plays 2. Nxf5, Black can’t recapture with his queen, for it is over on b4. Instead Black has to recapture with his e6 pawn, which in turn gives White a passed pawn on e5 that he can push to e6, threatening to promote it eventually and menacing Black’s knight right away. Black can deal with this (one possibility is Qe7, pinning the pawn; another is Qc5, attacking White’s queen), but starting with g6-g5 avoids these complications.