ISO Invariance

What in the wild world of sports is ISO invariance?
Definition, pulled from the web: The short answer is that ISO invariance means that a camera will produce the exact same image quality by staying at ISO (or whatever the base ISO is on the camera) and dramatically underexposing the photo and then brightening it up again in Lightroom, as if you had shot the camera at the proper ISO in the first place.
That would have been helpful to know.
Some time ago, a little over four years now, I had a post about shooting to the dark side.  Essentially, for all the wrong reasons and probably doing it wrong, I was on to something.  And I didn't know why.  My life is a blast!
I've often thought/known you can play with exposure settings in ACR, increasing or decreasing by what appears to be 5 stops.  Assuming you are using a RAW file.  For some action shots that I wanted to HDR, I'd make my own exposure range.  Wasn't always as good as taking three or five independent shots, but in a pinch I could get good results.
Back in the day when I thought shooting dark was the right thing to do, I was taking an ordinary day and using the exposure compensation to darken the image.  Clearly not understanding anything, I was kind of doing it right.
Since the MIHA shoot, I've been thinking on how to get better quality images either through software or new hardware like a faster lens.  So now there is another option, technique.  And it has a name.  Frigg'n awesome.
So here's how it will go.  At the MIHA for me to shoot, without flash, a 1/500 sec shot at something around f/5.6 that would be in the area of ISO 10,000.  Cleanable, but would clearly like to be a lot lower.  Knowing that I can add exposure of 5 stops, that would mean I could set my ISO at 640 and still save the image.  OK, that's to the edge but you get the point.  Shots I took at the MIHA at 1000 ISO were more than OK and cleaned up very easily.
So tomorrow there is another horse event close to home.  Seriously, less than 5 miles from the house.  It will be indoors.  Hopefully warmer than the MIHA, but the light will be bad - or worse.  So if I can remember what I learned today I have something to work on.
So the plan will be to initially set the camera to 1/500, f/5.6 and auto ISO.  If the metered ISO is around 10,000  I'll set the fixed ISO at 1200 - and start working my way to ISO 800.  See what happens.
With any luck, I'll learn something about this invariance thingie.  And something more about the horse world.  I should probably learn to spell equestrian.