It challenges the learner to listen for and then “speak” the missing words in the video. Very much like the old “cloze” exercises that teachers used to prepare for songs, dialogues or many types of audio in the classroom. Only difference here is that we do all the work for you AND you can speak the missing words instead of writing them.
Try it out. Just turn it on at the top of the player and you are set. Can you do the hidden challenge?