Spedding and his colleagues think that the unique aerodynamic wake signatures of bat wings are caused by different mechanical operations in the upstroke of the wingbeat.
Hummingbirds however, turn their wrists during the upstroke, changing their wing's angle to flap in a figure 8 pattern that continually generates lift throughout their wingbeats.
The bot only weighs 175 milligrams, about the same as a raindrop, so to keep it from losing control, they added underwater stabilizers, and slowed down the wingbeats.