been thinking about this some more… from a purely image processing perspective
normal spherical blur results in distorted information reaching the retina. This actually must result in LESS USEFUL information content of the light rays for the brain hence the brain cannot hope to reconstruct the missing information purely by image processing. This is why, I think there is some consensus on this forum that normal AF MUST be doing something physically with the cornea/lens/front of the eye - many people can even feel something happening there - and this allows the slight improvement in the path of the light rays hence improving the physical information quality reaching the brain in the first place.
At the same time, astigmatism often results in double vision - ok it is still a kind of angle-specific short-sightedness modifier - so you DO get some loss of useful information, but it is only partial compared to spherical blur - e.g. where you get (assuming sph blur is corrected) the correct information on some meridians. This COULD allow the brain to use the correct information in 2 ways: some meridians provide good light signals/information and if it manifests as double vision - you just get 2 of the same thing - this type of error COULD be eliminated or compensated for by image processing in the brain - it has enough information physically (“I know what the thing looks like, just gotta filter out one copy of it” vs. “I don’t even have enough information to know what it looks like”).
This would imply that it is not AF that helps astigmatism, but as suggested on this forum, also by e.g. @kem, that practice or training of the brain to fuse the double images is what helps it to learn this kind of image processing to improve on the negative effect of astigmatism on images.
Probably forgetting a ton of things here, but I don’t see how AF, as we experience it helping spherical blur, could compensate for astigmatism unless your ciliary muscle could bend the lens only in certain meridians, which I doubt…
…and this would mean that when practicing AF, it is not that which is improving cyl, but your focus/engaging your brain at the same time as AF-ing, teaching it to fuse doubled lines