An S1 is a cross of a plant back to an identical clone. To do that intentionally, one of the two clones has to be treated with chemicals to force it to develop in the flowering phase as a make. This allows the breeder to pollinate the other clone with the same sets of chromosomes.
The goal is to create seeds that have a reasonable chance of equalling the desired traits of the original clone. It doesn't always work out that way but there's a reasonable chance if you grow enough seeds out
Plants sometime develop both make and female organs, causing fh to self pollinate and create accidental S1's. that how you can get seeds sometimes when you're only growing one plant or one type of clone
To add to that, the plant being reversed can pollinate itself. Don't necessarily need to have another clone to pollinate. There's also another method I think Soma dubbed "rodelization"(not sure about spelling of it). It's where you let the plant go was past peak maturity. The plant will throw male flowers in a last ditch effort to ensure species survival.