Aye that is true, and highly annoying. But this method has been used many times before. I use it myself. Though they do sort of differ between painter its all the same principle.
Its a plain Black Primer is sprayed on first all over the model.
Then secondly from the direction of the light you spray a Grey Primer on, not as heavily this time as you still need the Black in some areas, it shoudl be short sprays that covers most of the areas facing the direction of light. Also when i do this I move the can around ina circular movement to lightly hit other areas not necesarily right in the light. So realy anything pointing downwards or directly to the opposite of the light should be black. Then the bits that shoudl be msotly grey are uward facing and facing directly towards the light.
Lastly get a White primer and spray lighter than the grey primer on the miniature mainly facing towards the light, and ofc other areas the same as the Grey primer. You want this coat to be quite light so it only realy hits onto the top parts/extremities of the model, so it doesn’t go too deep into the recesses.

Hope it helps 🙂
but as I said this is just my way, different painters ahve different methods.
The main reason for doing it this way as well is because it shows you which parts of the mini are hit by light more, and which have shadows. And the white gives a natural highlight, and the black gives a natural shadow. The grey is just to make more of a blend and transition into the shadwos and highlights, plus is a midtone to show where natural colour should be layed, as the light is not totaly getting there yet it is not a shadow because some light is getting there.

I hope all of that made sense haha 🙂