drukenknight: There was no good reason for him to exchange the B in order to move the N. Count the number of pieces on the 10th move that are developed. Black is basically behind by one piece. Whites Q is out there too but it is not ideally placed, black's Q is ready to move so he doesnt have to worry about getting the Q out right now. So black needs one more move to develop. Does he have time to do that or would he lose material?
So you look at the N on c3. N is fine on on c3 if bxc3 then black has 8 connected pawns vs 5. A N for 3 pawns what could be more equal?
Aside from the problem on the diagonal, moving the N again when behind by one piece is almost certain to mean you will fall further behind in development.
If you want to fall behind in development by TWO TEMPOS instead of trading a N for 3 pawns and having as many pieces out there as white, fine go ahead. YOu will get killed.
Far better to trade the N for an equal amount of material, and then you can catch up on development. Black is not that far behind in development because white's extra move with the Q is not killing him yet.
10...Nc6 11 bxc3 cxd4 12 Nxc6 Bxc6 13 cxd4 Bb4 something like that.
"Oh no DK, you didn't really get 3 pawns for the N, white still has more pawns than that. Besides you dont' want to part with a N this early in the game..."
Whatever, believe what you want to.