notyetagm: Here Black makes an -incredibly- instructive blunder with 9 ...  f6xd5?.Position after 9 ... f6xd5?:
click for larger viewLet's see what Martin Weteschnik says in the chapter on <PINS> in his great book "Understanding Chess Tactics", page 48. <
*) Every undefended piece is a potential candidate for a pin*) Every attacked piece of yours standing in front of another of your pieces should be considered as pinned *) Two pieces of the same colour on a diagonal or file should already be regarded as a precondition for a pin >
Now we see why 9 ... f6xd5? is such a blunder: it creates all the preconditions that Weteschnik indicates lead to a <PIN>! The Black b7-bishop is <UNDEFENDED>, the Black d5-knight is attacked and positioned in front of the <UNDEFENDED> Black b7-bishop, and the Black d5-knight and <UNDEFENDED> Black b7-bishop are <LINED UP> along the a8-h1 diagonal. So naturally the tactical genius Karjakin (White) sees all of these tactical points and plays 10  c2-e4!, <PINNING> the already attacked Black d5-knight to the <UNDEFENDED> Black b7-bishop. Position after 10 c2-e4!:
click for larger viewNow the Black d5-knight is <EN PRISE>, it cannot be defended again, and if it moves then the <UNDEFENDED> Black b7-bishop hangs. A very powerful <PIN> indeed. This may all seem rather obvious but remember that Black here was Germany's youngest GM who totally overlooked the combined effect of all of these tacitcal nuances which the tactical genius Karjakin did not miss. | 
