By the threat of 2...Bd3 Black provokes c3-c4, which will soften up the d4 and b4 squares for his pieces.

2.b4 Bd3! 3.Qb3 c4 , fixing a weakness on c3 and transforming the bishop into a giant.

Now that the plan with ...Nc6-d4 isn't feasible, Black directs his attention to the e4-pawn.

Preparing ...Bc6 and ...Qb7, with a slight but enduring advantage thanks to the bishop pair and the superior pawn structure.

By provoking c2-c3 Black leaves the enemy knight without its desired destiny.

Too materialistic.
White should have returned the pawn by 4.b3 Bxe2 5.Rxe2 Qxc3 just yields Black a minimal edge.

Now the active bishop pair more than compensates for the pawn, whereas White keeps having a problem with his misplaced knight.

This attack on the knight forces a minor concession from the opponent.