Why underpromote? Hjartarson vs Andersen

Question

Chess.com - Praggnanandhaa wins 2022 Reykjavik Open:

he missed a unique underpromotion to a bishop as the only move to win the position

Why does a Queen promotion not win him the game?

Answer 1

Why does a Queen promotion not win him the game?

Because black can play the kamikaze Qg2+ forcing stalemate after Kxg2.

Here is the critical position in the game where Hjartarson could have forced through the pawn to promote via the queen sacrifice Qf7+ but needs to promote to a bishop. Hjartarson instead played e7 leading to a draw. Presumably he calculated the queen sacrifice and saw that promoting to a queen led to stalemate but didn't calculate the underpromotion.

[title "Hjartarson vs Andersen Reykjavik Open 2022"]
[fen "4r1k1/3R2p1/2q1P2p/5QnP/5N2/6P1/5P2/6K1 w - - 0 1"]

e7 (1. Qf7+ Nxf7 2. exf7+ Kh7 3. fxe8=B! (3. fxe8=Q? Qg2+ 4. Kxg2=))