Braulio explains the concept behind using the twist sweep attack to counter an opponent's attempt at flattening you in half guard. He points out that in order for an opponent to pass your guard, they will generally need to follow a fairly precise path. Anytime you can divert them from that path, you will hinder their efforts to pass.

If your opponent forces you into reverse half guard and establishes a reverse underhook, the first thing you need to do is prevent them from working their underhook high into your armpit. You can do this by closing your elbows as they are transitioning. Once you have addressed their positioning, in relation to you, use the twist sweep to move their weight off of you and create space. With this space, Braulio shows how to attack the kneebar or transition to the anaconda guard.

Links to other Parts: Part 1, Part 2