Walking onions grow tall stalks that are crowned by a bunch of bulblets. The weight of the bulblets bends the stalk over and whenever a bulblet touches the ground it takes root. Hence, the appearance of walking across the garden.
Catawissa Onions are different in that they form the crown of bulblets, then a second tier of bulblets form on the tendrils from the crown.
Here are a couple of images that show the current stage of my Catawissa Onions:

