When you self a plant you cause many gene pairs to double for example (AA).
By evaluating the progeny you can see how homozygous your p1 was.
If your s1 phenos are all over the place you know your p1 is a polyhybrid or heterozygous for a lot of traits (Aa) like a mofo.
We would consider like or homozygous gene pairs (AA or aa) to be true breeding for that gene pair thus to some extent the trait it controls.
When you cross an s1 back to the original mother plant P1 any gene pairs that they do share should then double up to a homozygous state and the subsequent progeny will be true breeding for that gene pair.
It is a good tool for many uses such as isolating genetic building blocks and selection.
Whatever comes out of that s1#100 x the sour dubb, you can be sure you will find phenos that will be homozygous for many sour dub gene pairs/traits. The progeny of those will 100% pass on traits reliably to their future children.
Traits that express from c-dominance or incomplete dominance add another wrinkle to complexity of it all heh...
I didn't make any sdubb x sdubbs1. I don't have any plans to do that. I only want s2 at this point