A "horse stance" is simply extending both glutes at the start of a spar and keeping your chest held while doing so, if the arms are moved downwards you should generate momentum in the upwards direction for the whole tori itself allowing your legs to spread out in -30 gravity (Lower shoulders or something d00d).
It is essentially a way to "balance", but I would not recommend doing it unless you really need to, as
1. It's boring and overdone to death
2. It can cause an issue with flow and generating momentum, as you're essentially getting stuck in one spot with your legs out wide.
3. This game is heavily situational and it can look horrible in some positions