When a response is one in which you believe it would spark further discussion, this is where "spark responses" comes into play.
To the right of the response you will see the Actions button. Select that and then choose Spark Response. From there you you will have the ability to create a Topic using the student's video as the Topic Resource. You will be able to edit the settings for this new Topic once you select Spark.
Embedded below is a GIF followed by a 1 minute and 18 second YouTube tutorial.