These start out normal - you plant a little set-sized onion and you get a normal onion-planty green stalk. But if you let that stalk keep going, it will eventually grow another little set sized onion on top, up in the air. Soon to be followed by more little onions.
If you let these little onions grow for long enough, the weight of them will bend down the stalk of the parent plant and the onion will land in the mud and sprout. This starts the whole cycle over again.
You can, as I understand it (I haven't done this yet), eat the in-the-air baby onions, or you treat them as onion sets and eat their green stalks.
To see a picture of these guys, look here.