How do they work?

Yes.
Astigmatism (strictly not *an* astigmatism) is only a variation on short or long sight.

An eye needing a correction but without astigmatism has one number: -2.75 , for example. That would convert a dot target in the distance from a blurred circle back to the desired sharp point.
Now with astigmatism, the eye needs more correction in one orientation than another. -2.75 might reduce that dot target to a line, of give a letter E clear horizontals but a blurred vertical. In focus in one direction, but needing more focussing in another, without touching the direction that's already sharp.
Hence the three numbers of an astigmatic Rx: the amount ot get one meridian in focus, the extra amount for the one at 90 degrees to that and the direction in which to apply it, since it doesn't have to be horizontal and vertical.

Now in the eye a standard contact lens tends to spin a few degrees every blink. But if the prescription is the same every way up, this doesn't show or matter.

With an astigmatic contact lens, not only does it have to have the two Rx strengths it has to keep the "right way up".
There are two main ways of doing this, but they tend to make the lenses thicker. Finding the lens that is comfortable and stable in the eye can take more trials than with a non-astigmatic lens.

First - contacts don't have astigmatism - people do.
Contacts for people with astigmatism exist, I wore some for a long time. I started on hard lenses (gas permeable), but they were impossible for astigmatic eyes. I finally went to soft (toric) and they worked, but they were very expensive. It's impossible to make toric lenses disposable. I lived with those heavy lenses on my astigmatic eyes until I had surgery on my ocular muscles to correct strabismus. My vision was surgically corrected with in-situ keractotomy a couple years after that.

To explain astigmatism, think of your eyeball, specifically your cornea, as a grape. It's curved, and the light refracted throught the pupil onto the surface of the retina gives the brain the sensation of the objects in front of the eye. Wavelengths of light provide the color.
In an astigmatic eye, the surface of the cornea (in front of the iris is the area in question). Instead of being completely smooth like a grape, it's a little more similar to a raisin, or if not as exaggerated, not perfectly spherical. This causes that the light does not reach the retina in the same way, causing visual percepation that is not as clear to the brain.

My contacts designed for astigmatism were weightier on one side. The heavier edge would cause the lens to rotate to fit the specific needs of my eye, as it was not completely smooth. Now my eyes are perfect and I am thankful for that every day when I wake up and see the patterns of light on the ceiling.

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