notyetagm: 40 ... g4! is a <double attack> against two loose pieces. Before this Black queen move both the White e4-bishop was loose (0 attackers, 0 defenders) and the White f3-rook was loose (2 attackers, 2 defenders). Then when Black played 40 ... g4! he simultaneously added an attacker to each of these loose White pieces, putting them <both> en prise at the same time: the e4-bishop now has 2 attackers, 1 defender while the f3-rook now has 3 attackers, 2 defenders. White has no adequate way to meet this double threat and so resigns.

Quoting Dr. Nunn's famous saying, <LPDO - Loose Pieces Drop Off>. In particular, having two loose pieces on the board (e4-,f3-) while the enemy has his queen is an invitation to disaster (40 ... g4!).

Yes, White has many ways to meet the threat to his f3-rook and he has many ways to meet the threat to his e4-bishop. <But White has no satisfactory way to meet both threats at the same time.> Like they say, tactics are based on double threats: you can meet one threat or the other easily enough but not both simultaneously.