RT 5.4-rc2 - Soft-proofing / Out of gamut

Could someone explain about how the cyan OOG marking works please?
I opened an RT_sRGB Jpeg thinking there would be no OOG marking, but there was, and quite a lot of it. This was with RT_sRGB output and monitor setting. So I guess the marking starts before the limit is reached, like over-exposure I suppose. But the question is, can you the dev(s) give a feel for how soon it starts to register. I tried reducing saturation and changing hue (LAB adjustments) and it took large changes to remove or minimise the cyan in some jpegs. One was nearly a black-and-white before the last cyan disappeared, I was surprised!

I suppose what I’m asking is around this - you want to have a feel for what you can get away with re. OOG colours, which I appreciate doesn’t have a simple answer, but it would be good to know how the cyan marking works.