If I were making one of these, for the dip I would use either a servo (which I have on hand) or a small motor. The trick is build her with a "flexible spine" . The groom simply bends in the middle and is attached to the bride. To make the bride stand up straight a string is pulled from below. To make her lean backwards the string is relaxed. The groom just follows her since he is connected by his hands. Basically like this but even more simple as the bride only needs one degree of motion. The "dip" is not connected to the rotation, and can be triggered anytime.