The video depicts a bat flying in a wind tunnel during simulation. The trained bats flew through thin smoke to reach a stick with food on it. Meanwhile the researchers aimed a laser beam at the smoke behind the bats and took pictures of the illuminated smoke particles. The researchers measured how the smoke moved to calculate the forces generated by each beat of the bats' wings.
Learn more about the experiment in this video: https://www.youtube.com/watch?v=YhEkperhZMQ