it's a specialty foot made especially for sewing curved pieces (i.e. drunkard's path) to one another. by holding the fabric correctly, the foot does the work of gathering in the convex layer to fit the concave layer without using pins or having to snip.
You still have to snip after you've sewn the pieces together. I have one of these and could never get it to work. I'm trying to sell mine if anybody is interested.