|notyetagm: White To Play: 20 ?
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20 xf6+! is a very nice <DISCOVERED DEFLECTION>: use the masking piece (White d5-knight) to take what the newly overworked piece (Black e7-bishop) no longer defends (Black f6-pawn).
The tactical point is that if the White d5-knight were not blocking the 5th rank, then the Black c5-queen would be loose, attacked by the White e5-queen and defended only by the Black e7-bishop. So if the White d5-knight were not blocking the 5th rank, the Black e7-bishop could leave the a3-f8 diagonal only on penalty of losing the Black queen. Hence if the White d5-knight were not blocking the 5th rank, the Black e7-bishop would not count along the d8-h4 diagonal.
So the White d5-knight takes on this newly undefended diagonal with 20 xf6+! and Black cannot take back with his newly overworked e7-bishop because it would drop his queen (20 ... x?? 21 x).
<Discovered attacks that make things loose or pinned are dangerous, dangerous, dangerous.>