Half guard sweep and defense

So I have been caught on the same sweep a few times.

I do not know the name, not sure how to set it up or how to defend it.
I searched for it online, but unfortunately I do not know the name and "half guard sweep" on google returns way too many results.

So I will try to describe it here in hope to get two things:
- A video or instruction on how to actually apply this sweep
- How to defend it

This is how it usually goes:
- I am on half guard
- My opponent pummels and underhooks
- I wizzher the arm to avoid having my opponent take my back
- Then my opponent does some magic movements with his legs and starts putting pressure on my knee forcing it to collapse
- There is enough pressure on my knee that I either accept the sweep or fight it and run the risk of injury


I understand that step one is to avoid giving the underhook, and I have a few ways to get out of the underhook if I lose it, but I would like to learn how to do this move and how to defend it without running the risk of injury