Tal meant to play a6 three moves ago and put a knight on f6 instead. Fischer's knight came to b5 with tempo, the black queen was pushed back to b8, and the knight on e5 is now pinned against it by the bishop on f4. White wants the queen on d4 next. Played at once, Qd4 loses her.
Diagram 33 · White to move
Fischer vs Tal, Bled International Tournament 1961