Defense is often a neglected aspect of BJJ. When you see highlight videos, you most often only see athletes successfully submitting or sweeping their opponents. But really, a lot of the time they are able to do so because they have so much confidence in their defense.
Roger Gracie is known for having one of the best overall sweep/submission defense in all of BJJ and even states in interviews that he is able to succeed because of that.
Now a very slick triangle defense by UFC champ Georges St. Pierre