Position after 9 ... ♘f6xd5?:

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Let'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!:

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Now 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.

Yes, the problem with 10 ... ♘d5-b4? is that the b4-square is -not- an <OUTPOST>. After 14 a2-a3, the offside Black b4-knight is left with <NO RETREAT>.

This is yet another example of a tactical theme which I call: <YOU SHOULD NOT GO FORWARD IF YOU CANNOT GO BACK>. You just end up getting trapped, probably on the enemy's side of the board where you do not belong unless you are in an outpost.