Tomato plants come in one of two types: Determinate and Indeterminate.
A Determinate plant knows what it will be when it’s a seed. It’s gonna grow into a bush, flower, fruit, wilt and die. An indeterminate plant isn’t an organism, it’s a program. A stalk will emerge, it will grow a stem, from the stem will arise an alternate pattern of leafy branches and flower clusters. At the crotch of each leafy branch will emerge a sucker. That sucker will grow like a full plant, alternating leaf flower clusters and leaf branches, with their own suckers.
Tomatoes also have an interesting characteristic: Anywhere they touch soil, they’ll put out roots.
So pinch a sucker off a young tomato plant (part of pruning them) and then stick that sucker in some fresh, moist soil. It’ll put out roots and become it’s own plant, genetically identical to the original.
Took 5 minutes, most of it walking.
I don’t think there’s an exact science. I tend to take the first couple from the base of the plant, I also take off the first flower cluster. I want the plant to put its energy into growing big early in the season, not worth a couple early tomatoes. From there I pretty much let it grow.