The first step to spin a true woolen yarn is to card the fiber into a rolag using handcarders.
The passive hand smooths yarn, picks out vegetable matter, and pulls out extra bits of fluff,[1] but that is all.
The work of drafting is done by the active hand, and most of the regulation and smoothing of yarn is done by twist and tension.
Drafting is done by pinching off a short section of the rolag, and then pulling back while twist is added to it.
This has the effect of automatically thinning out the thicker parts, which is what allows this technique to work.