The bird claps the trailing edge of the wings, and leaves a small pocket of air at the leading edge. As the bird flaps down, the pressure at the trailing edge drops, pulling the pocket of air at the leading edge. The under side of the wings create high pressure by capturing some air, the high pressure air bleeds into the low pressure pocket created on the downbeat. Two vortices are created behind the bird. On the subsequent upstroke, the bird positions its wing to take advantage of the vortex. You can see the vortex lift, what the people in the video refer to as ripples in the primaries. The bird positions its wings to ride the vortex which creates lift in effect sucking the bird's wings together on the upbeat. It's physical energy spent mostly on the downbeat, as it rides the vortex on the upbeat.