A strong pawn sacrifice, which creates a passed pawn on the f-file and enables a blockade on e5.
More precise was 4...f2! first, e.g. 5.Qe3 Ne5 6.g5 Qf3+ 7.Qxf3 Rxf3 with a clear advantage – Ni Hua.
After breaking the white blockade, Black is able to move his queenside majority forward.