I have played a couple of games as black with this line. It is a simple plan: to put pressure on the weak c3 and c4 pawns.
This line is mainly known as the Dzindzi-Indian Opening, according to the comments and this video. It's also called the Modern Defense: Pterodactyl, Beefeater Variation according to chess.com.
[FEN ""]
1. d4 g6 2. c4 Bg7 3. Nc3 c5 4. d5 Bxc3 5. bxc3 f5
I have tried it while playing correspondence, so I have followed the move suggested by databases: 5. f5. This has had good results after quickly playing Qa5, trying to attack the weak pawns along with other minor pieces.
5...f5 347
2018 2403/2426/2709 30.3% 25.6% 44.1%
5...Qa5 22
2017 2359/2306/2586 50% 22.7% 27.3%
5...e5 5
2014 2370/2338/2418 40% 20% 40%
5...d6 1
2010 Steadman vs. Bosboom 0-1
*Chesstempo database for the position
I wonder what the reason is for taking away a tempo to play f5 while the plan is clear, at least as I play it: to profit from the doubled pawns by attacking on the queenside.
Is f5 played to avoid e4?
f5
is mandatory to make e4 a bit more expensive for White (it's still a good move).