|notyetagm: White To Play: 22 ?
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The Black e6-pawn must meet the threat of the royal fork ♘b6-d5+ by <DEFENDING> the d5-forking square. Since <DEFENDING> this square is the defensive task being performed by this pawn, Black cannot also use it to <BLOCK> the e-file.
Black must <DEPEND> upon his e5-bishop to <BLOCK> the e-file, meeting the threat of ♖x♔. But since the Black e5-bishop is then busy <BLOCKING> the e-file, it cannot also <DEFEND> the c7-queen.
So the Black e6-pawn <DEFENDS> the d5-forking square while the Black e5-bishop <BLOCKS> the e-file.
Vescovi's 22 ♘d5+!! magnificently exploits this tactical situation. After 22 ... exd5, the Black e5-bishop is <PINNED> to the Black king, allowing 23 ♕x♕+ and 24 ♖x♗.
Black's problem is that the Black e6-pawn must <DEFEND> the d5-forking square, the Black e5-bishop must <DEFEND> the Black c7-queen, -and- either the Black e6-pawn or the Black e5-bishop must also <BLOCK> the e-file. So one of them is <OVERWORKED>. That is, Black has -three- defensive tasks to perform (<DEFEND> d5-square, <DEFEND> c7-queen, <BLOCK> e-file) but has only -two- pieces (Black e6-pawn, Black e5-bishop) with which to perform them.