Surrendering the d4-pawn in order to create a double threat at g4 and b8.
Unlocking White's queenside in order to enhance an attack on the exposed king on h2.
5...Qxb6 6.Na4 Qb5 7.Nc3 Rxc3! 8.Bxc3 Qe2+ 9.Kg1 Rxc3 10.Qf2 Qxe3 11.Qxe3 Rxe3
The simplest follow-up, in order to recover the pawn at once, after which the bishop pair becomes terrific.