Now Black will never be able to recover the pawn; but after 4...Nxe4 5.Nxe4 Bxe4 6.Qh6! White would have a strong attack.
2.cxd6 Bxd6 3.Nc5 was a better choice, when 3...Bxc5 4.Rxc5 h4! keeps the initiative – Karpov.
Softening up the g6-pawn (and the black kingside as a whole) for a later attack.