In the Blackmar Diemer "Vienna defense", black does not take the pawn on f3, but rather defends it through the bishop 1.d4 d5 2.e4 dxe4 3.Nc3 Nf6 4.f3 Bf5.
Now white has three important alternatives :
1. attack like a madman with 5.g4 Bg6 6.g5 Nd5 7.Nxe4
2. attack like an even bigger madman with 5.g5 Bg6 6.h4
3. play quietly with 5.fxe4
The first option (diagram) used to be most popular in Diemer's era.
The great point of this "Hara Kiri attack" is that white can set up some terrible difficulties for black, eg after 7...Nc6 8.c4 the dreaded "The Bouver attack" appears and the line 7...e6 8.c4 Bb4+ 9.Ke2 is not easy either.
The problem is that white has to set up his pawns like a complete beginner. Most positions in this line seem quite strange, with white advancing too many pawns. A computer might be able to handle that, but lots of humans will feel uncomfortable with the resulting positions when playing white.
The second line was recommended by Scheerer in a must-win situation.
I dubbed this line the "More than Hara Kiri". Scheerer now suggests 6...h6 7.Bg2, but 7...Nc6 seems to give black a clear advantage.
The last line 5.fxe4 is the simplest and was covered in my book "Attack with the Blackmar Diemer"
Black has two decent replies 5...Nxe4 6.Qf3 and 5...Bxe4 6.Nxe4 Nxe4 7.Bd3, both leading to equal play.
Even if I like attacking like a madman ( I play the Blackmar Diemer for a reason !? ), the "Hara Kiri" and "More than Hara Hiri" just see too much for me - I will stick to the boring 5.fxe4 for now.