Black is in danger of seeing some large pieces fall off the board in the next few moves. In addition to the piece currently en prise, the f3-knight is threatening to take on d4 ganging up on the c6-knight and freeing the f-pawn to advance to f3 and win the house.
Black has to start some sort of counterattack, and the way to do this is with 7...d5 ignoring the pin on the c6-knight. 7...f5 is too little too late (8. Nxd4 b5 9. Bb3 Qh4 10. g3 Qf6 11. f3), and 7...Qe7 swaps one pin for another (8. Bxc6 dxc6 9. Nfd2 Bf5 10. Qf3 just about wraps things up). Anything else drops the knight.