So I was watching the little show Wanderlei put on before the Real Deal. During the video, Rafael Cordeiro gets him in side control. Wanderlei then purposely underhooks the near leg. I've never liked doing this, because you can get your arm trapped, and then kimura-d Matt Hughes style. Or in an MMA situation, you will just get hammerfisted with only one hand to attempt to defend yourself. I understand he can use it as leverage to throw him over, but first he'll need to get his weight off of him. What am I missing here? What is the big advantage?

I normally bring my elbow into their hip, and then go for the underhook on the otherside.

I haven't seen the show in question and I'm not an MMA fighter, but it might be to prevent knees to the head.

I'll do it because it can help in getting a half guard. I don't normally spar MMA though, so I'm not sure if it would be a good escape under that ruleset.

Possibly to stop the knees, or he may have just started that way so that he could push into the guy's arm pit with his other hand, to get the guy's weight off him (for example this is used as an x-guard entry). Maybe then he can create space to stand, or get his guard back.

I sometimes do this to hold their leg in a position where I can heel drag them into my half guard/a sweep.