The feet are gathered up into the right hand while continuing to hold the bird upside down in the left hand.
The upper legs are pinched between the first and second fingers. The thumb is pressed onto the lower legs to keep the birds well pinned down and so they wont twist and break their legs.
The thumb presses down on the legs, holding the bird securely. The rest of the fingers are up underneath the tail feathers.  By rapidly moving the hand up and down, the bird will flap their wings.  This bird was not a happy camper by this point. Notice the closed eyes. It was fine when put back into the cage.