Sorry for the lack of a board here, I will learn how to paste boards here soon.
8.f4 b4 9.Na4 (Nce2 is reasonable too) Nxe4, one might ask, what has white got?
- O-O and black has serious issues developing as the amazing line given by the comp (I wouldnt have seen this line) :
10...Be7 11. f5 e5 12.Bxf7+! Kxf7 13.Ne6 Kg8 ( Qd7 14.Qg4 wins material back, Bxe6 fxe6 Kxe6 Qg4+ is no good, Ke8 Qd5 wins material back with decisive effect.)
14.Qd5 and Qh5 g6 (h6 Kh7 Qf5- Qxe4 wins) Qf3 win back material.
The safest line that black can go for is 10..Nf6 imo, just retreating, but white has interesting play with 10.O-O Nf6 Qf3 d5 (what else?) 12.f5 (full throttle) e5 (what else?) 13.Re1 or 13.Bg5!? are interesting for white.
The lines go on and on, but white has very direct, brutal play with the pieces working with the f-pawn in some shape or form. Whether it works or not is another matter.
After all, f4 isnt the main line, O-O is. Diagrams below are me visualizing my lines stated above.
1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 a6 6.Bc4 e6 7.Bb3 b5 8.f4?! b4 9.Na4 Nxe4 10. O-O Be7 11. f5 e5 12.Bxf7+! Kxf7 13.Ne6 Bxe6 14.fxe6 Kg8 15.Qd5
1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 a6 6.Bc4 e6 7.Bb3 b5 8.f4?! b4 9.Na4 Nxe4 10. O-O Nf6 11.Qf3 d5 12.f5 e5 13.Re1 e4!?
It's probably better it opt for something safe.