notyetagm: White To Play: 20 ?
click for larger view20  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.> | 
