I can see ( i think ) what you're confused about Barbara.
You're quite right, when trying to find the hidden fur of a starter the wtg is to use the most recessive fur there is , especially if it's given you reason to think it may be carrying an ultra recessive, and who knows maybe a new TR.
That way, if your starter produces this fur then it's a signal to start back breeding to the starter. The same applies of course to all other traits.
What we discuss here is a bit different though. I won't go through it all again, but this is in the case of buying a non starter hiding the most recessive new and showing the second most recessive.
We are just showing that it's a mistake to buy this kind of cat as if you only have the one, then you have an extremely small chance of ever getting at that new TR.
The probabilities of it passing as hidden are of course the same as with a more dominant fur, but as theres no fur in between the two, you can have no possible indicator fur to allow you to know if it's been passed.
And thanks to all you guys who posted so far, very useful and valuable contributions