Nope, it sounds like someone who is trying to be clever with colour but it still doesn’t make sense to me.
Why would you mix a highlift tint containing bleach and a small amount of neutralising tint with a lower level tint? The bleach will presumably wipe out some of the colour pigment in the 8 and then won’t lift as high as it ought to?
If you want something around a 9, you could mix an 8 with a 10 but I don’t see the point in mixing a highlift with any other tint.
I think this is something that probably works better with pure tone colour lines that allow you to control the exact amount of tone in your formula. With regular pre-mixed colour the results could be unpredicatable.
The lifting power in the colour is determined by the amount of ammonia in the mixture (highlift uses a higher ammonia% than regular tint). The downside of increasing the ammonia or peroxide level is that your colour deposit dimishes, as the excess oxidation breaks apart the newly formed colour molecules.
So if you were for example going to create a high-lift brunette, you would need to be able to increase the amount of ash in the fomula to account for the loss of deposit otherwise you would end up with brassy results.
What was the purpose of mixing the level 8 and 12?
As far as I know generally you can't mix high lifts with standard tint, because the alkalinity is different. Highlift is usually mixed 1:2 with 30vol or 40vol because of the ingredients.
This is why generally we also shouldn't use Highlift as a toner.
Having said all that, on a joico creative colour course the educator said you ca the achieve a strawberry milkshake colour by adding one of the red shades to your bleach.
Again - standard practice is that you also don't mix tint with bleach. But I suppose every brand is different.
Did the guy explain what you could potentially achieve by mixing the 12 with a regular tint?