A windmill (vitriak) is a cross.......in motion.  It is a svarha (swastika), an ancient solar symbol.  Svaroh, for who the symbol is named, was the god of the sky, the sun, and heavenly fire (thunder).  The svarha depicts the movement of the sun across the sky.  As Marusia Chaika notes, the swastika/svarha marked movement: good if following the sun, or evil if going against the sun.

The windmill/svarha was depicted in different ways across Ukraine. 

In Podillia, according to Selivachov, it was a rectangular design that resembled an actual windmill.  In Bukovyna, Halychyna, Volyn and Poltava “wedged” designs predominate, giving the whole sign the appearance of a Maltese cross. In Chernhihiv the vitriak is a stylized rosette with a notched contour.

(Note that numbers 3 and 5 are embroidery motifs, and 4 is a weaving motif.).  These examples only begin to hint at the varieties of expression of this motif. The first two are from Volyn; the one on the left shows a geometric version of the svarha, while, on the right, you have a more curvilinear version.  The latter design is called “Verba (Верба)” or willow, and the arms of the svarha have sprouted leaves.

These two pysanky from Podillia are both quite interesting.  The one on the left adds crowns to a simple svarha, turning it into a swirl of serpents.  The one on the right transforms the svarha into a botanical motif, its center now a flower, and the arms leaves.

The pysanka on the left is a simple one from Kuban, and a straightforward vitriak.  On the right is an example from the Chernihiv region; the arms have become a rosette with notches.

Binyashevsky has several examples of vitriaky in his work.  Most are of the rhomboid cross type, as in these from northern Bukovyna, Hutsulshchyna and Boykivshchyna:

And this simple one from Eastern Polissia:

He also has this example from Pokuttia, which is similar to the one from Chernihiv above, in that it is a notched rosette:

This example is of a curvilinear design; although Binayshevsky has labeled it “Magpies,” it is quite obviously a svarha:

More examples will be given in the photo album below.