With the threat of 22 P-Q5, so Black has no choice.
There is no defence, e.g. 22.. . NXN 23 RXP+ KXR 24 Q-R1+ or 22 ... Q-K5 23 B-B2 Q-N5 24 P-B3 Q-N6 25 N-B5 PXN 26 RXB +-.
There are so many threats at this point (even the simple R/4-KR4 will be deadly) that Black may as well grab at the exchange.
A beautiful dénouement. Just as everyone was waiting for execution along the twin B diagonals Steinitz reveals his true plans with this stunning surprise.
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