notyetagm: With 20  xb6+!, Keres rescues his cornered a8-knight:
click for larger viewThe variations are quite nice.
If Black captures the White b6-knight with his c6-queen (20 ...  xb6), then the Black queen is <DEFLECTED> from defending the Black c8-bishop and White (Keres) wins with the elegant <DOUBLE SKEWER> combination 21 f5+  c7 22  c3+. 
click for larger viewThe first skewer (21 f5+) goes all the way through and makes the Black c8-bishop loose, and then the second skewer comes (22 c3+) and goes all the way through to put the Black c8-bishop en prise. If instead Black captures the White b6-knight with his d8-bishop (20 ...  xb6), then the Black dark-squared bishop is <DEFLECTED> from controling squares along the h4-d8 diagonal. In particular, after 21 f7+ the Black dark-squared bishop is no longer available to interpose the queen check on e7. After the forced 21 ... d8, the Black dark-squared bishop is not in position to defend the f6-forking square and 22 xf6+ wins the undefended Black h8-rook. 
click for larger viewNote how Keres carefully analyzed the consequences of the <FORCING MOVE> 20 xb6+! to see what weaknesses this move <FORCED> Black to create in his position. This is the beauty of <FORCING MOVES>: they limit your opponent's possible replies. You can then do a detailed analysis of the weaknesses that result from your opponent's -limited- replies to your <FORCING MOVE>. | 
