This is the key! Using the fact that Black did not complete development, Black cannot protect the pawn and is forced to push it. Therefore White manages to create a weak square on c4 which is extremely tasty for a knight. Note that this works because taking on e4 will favor White thanks to tactics.

Another important moment. Where to go with the knight?

Now we have won the c4-square for one of the knights, which will put pressure on d6 in combination with the g3-bishop. You have two knights: one is going to f5 and the other to c4. That is why Nd1 is stronger!

2...Nxe4 3.Bd3 f5 4.Bxe4 fxe4 5.Rxe4 Ne5 6.Ne3

Finally a knight is arriving to c4 (Ndc4) and the black pawn on d6 will suffer a lot. The black king is also very exposed. Decisive advantage!

Boom!

Threatening d6 and the pawn on e5 at the same time.

and Black is almost losing. We see that ...Bxd5 will not work due to tactics.

And the queen is trapped!

The black king is pretty unsafe at the moment in the center, and White is threatening to start a pawn march with e5-d6. Giri employs an imaginative maneuver to block the pawns and at the same time restrict the bishop.