This is from Dick Raymond's "Joy of Gardening":

"Cabbage heads, like all vegetable heads, grow from the inside out. If yours start to crack, this probably means that the cabbages are growing too fast in the center. (This condition is frequently caused by heavy-handed fertilizing.) If you let the cracking continue, the head will split wide open and send up a seed stalk.
If you see a crack, hold the head and twist the whole plant halfway around, like turning a faucet. Don't be shy! Grab the plant and twist if halfway around. This breaks off many of the roots and that slows the inner top growth of the plant. Give the plant another quarter turn in a few days if the cracking contiues."

Disclaimer: I have NOT ever done this myself. I'm just forwarding it as strictly informational. Perhaps someone here who's actually performed this ritual can chime in. Personally, I'd be afraid of just ripping the whole cabbage head off.

Before reading the above post that is what I would have suggested. Don't know where I remember it from but I have done it. Surprisingly it didn't kill the cabbage plant.