So for whatever reason, if you have a config that says
It won't move the particle forward at all. It just stays in a vertical position, only offseting by 1 on the Y axis.
However, if you do so=1 in that, you suddenly see the particle move forward and off to the side by 1. If the side offset is very close to 0 for whatever reason I believe it forces forward offset to be 0 as well Image may be NSFW.
Clik here to view.
MythicMobs version: 4.2.0...
Sideoffset = 0, Forward offset also = 0
Code:
Skills: - e:p{p=flame;a=4;vS=0.1;hS=0.1;fo=4;so=0;yOffset=1} @self
However, if you do so=1 in that, you suddenly see the particle move forward and off to the side by 1. If the side offset is very close to 0 for whatever reason I believe it forces forward offset to be 0 as well Image may be NSFW.
Clik here to view.

MythicMobs version: 4.2.0...
Sideoffset = 0, Forward offset also = 0