With your back to the ground you have a perfect root.
When you are on top, it is very easy to give all of your weight to your opponent.
If you can consistently have a solid root in fixed step push hands, and give all of your weight to your opponent to carry when you are just standing in front of him, then doing those to things on the ground is remarkably easy.
Push hands is great training for protecting the head through continuous attack. Great for learning how not to get locked up.
As you get better in push-hands if your opponent tries to get under you, you let them try to carry you in a position where they have no leverage. If he tries to get on top of you, you float him. (both just like ground fighting, but harder)
As you get better still, you practice moving a heavier opponent from the worst possible angle, that's why push hands needs to be practiced at a slow speed.
And when you have reached a level of skill where you can express power without having any root and completely melt your structure you can increase the chaos for your opponent until they simply defeat themselves.
In Brazilian Jiu Jitsu and Wrestling the goal is one-on-one domination on a soft mat. In the case of a surprise attack by a heavier opponent or multiple opponents where you go down to the hard ground! --instead of trying to dominate and control the situation, you want to increase the chaos, and keep rolling You want to keep as much momentum in the fight as possible--and keep up continuous striking the whole time.
Fixed step push-hands and roushou is one of the best types of training for this situation ever invented.
Rory gave a rule of thumb which he explained like this. If I am fighting on the third floor balcony of a condo and I'm about to die in a choke hold, I jump off the balcony with the guy choking me.
Rory's rule of thumb: When you are losing, increase the chaos. When you are winning, gain control.
I have a corollary to that rule: If your opponent is experiencing chaos and you are comfortable with it, that works too.
A lot of training in internal martial arts is about creating disorientation and relaxation at the same time-- Or perhaps I should say unusual orientations like spinning in bagua, or unfocusing the eyes.