Sounds like what the Chinese call ming men.
Would you care to elaborate on that? How is weight underside connected to Koshi?
Is that the same 'ground force' as Mike Sigman's ground path?
How can I tell if I do that correctly?
Ming men? I asked Mike if this was so and he said no. However, not that I'm too interested in what the Chinese call it or if their view on it is the same, but I believe they call it 'yao'
Weight underside: When practising weight underside, admittedly until you get good at it, you are practising relaxing the body, letting Ki flow through the body, learning about centre etc. but at the same time becoming more and more aware of gravity as if you are actually harmonizing with gravity.
Eventually you feel the difference between this harmonizing with gravity and centre. You feel and know the difference.
Now actually I'll tell you what happens. You feel what seems like a door opening at Koshi. Where most if they check it will find the base of their back tightens this is the complete opposite. Instead of tightening like a door closing, it opens and you feel Koshi. Now you realize that when you lose weight underside it is Koshi that is not open or relaxed. Get Koshi back and weight underside returns.
Now when you are used to and using Koshi all the time you find weight underside is there all the time and no longer a consideration.
As to wether that's the same ground force as Mike or whoever I don't really care to be honest.
To tell if you're using it properly is obviously a matter of practice and thus seeing the difference of when you are and when you are not using it.
Rather than go on about what it does etc. I'll just give you an exercise to do so that you can feel the difference.
Try doing funakogiundo with a partner pinning your wrists to your side. Try it with force. Try it with centre. Try it with Koshi. See the differences. Of course I am assuming you are familiar with these exercises and the nuances of them . Good luck.