Why is 5. f5 so necessary in the Dzindzi-Indian Opening/Modern Defense: Pterodactyl, Beefeater Variation? 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. 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 correspondance, so I have followed the move sugested 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&nbsp; 347
2018&nbsp;&nbsp;&nbsp; 2403/2426/2709&nbsp;&nbsp; 30.3%&nbsp; 25.6%&nbsp;&nbsp; 44.1% 

5...Qa5 22
2017&nbsp;&nbsp;&nbsp; 2359/2306/2586&nbsp;&nbsp; 50%&nbsp;&nbsp;&nbsp; 22.7%&nbsp;&nbsp; 27.3% 

5...e5&nbsp; 5
2014&nbsp;&nbsp;&nbsp; 2370/2338/2418&nbsp;&nbsp; 40%&nbsp;&nbsp;&nbsp;    20%&nbsp;&nbsp;&nbsp;&nbsp; 40% 

5...d6&nbsp; 1
2010&nbsp;&nbsp;&nbsp; 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?
You don't want a White pawn getting to e5, especially with a friend at f4.  White can let you have a c-pawn if he can achieve such a central dominance.

Also, without f5 it is awkward to develop the Ng8 -- you notice none of the variations you show have Nf6.  But now after f5, Nf6 will grip the e4 square.   Thus White often plays e4 anyway!