PGB21

Position after White 21  

The urgent area of interest is the top left corner.

Moves 22-31  

White 1 connects the stones as the white group tries to ecape from pressure.

Black 2 is a sacrifice and perhaps a mistake.

White 3 is a huge mistake. White had to play at 4 and was going to: but suddenly hallucinated that four stones are about to be captured.

White 3 left the situation unsettled but equal  

Back 4 is an atari, so White has to capture with 5. Black takes the white stone in a ladder with 6 and White has to survive, forcing with 7 and then adding the second eye with 9. White is alive, but only just, in gote. Moreover, Black 10 captures a single white stone and gets a ponnuki. This is painful for White.