I believe this is a ruffler/pleater (many of these feet do both functions). If you put the first fabric in the slot, and a second fabric on top, both will be ruffled. If you put one fabric under the foot, only the top fabric (through the slot) is ruffled.

I would love to win this - just wish it made more than one size bias tape. The 1 inch is fine, but when I applique, I usually use much thinner bias for vines, etc.