In this video Chico Mendes demonstrates a leg lock from the "50/50 guard". This "guard" is actually a Sambo lower half submission fighting position. You can learn more about Sambo by Clicking Here !
Basically, what he is doing is a Double Achilles lock when the person Triangles their legs to secure the position. In the video he does more of a calf crush but it is much more effective if you break the Triangle by extending the persons legs and then wrap their ankles up for the finish.
All that has to be done quick because from this position you are in danger of getting your miniscus torn by a heel hook or at least getting tapped by an Achilles lock if your opponent goes easy on you.
Sorry, this video is in Portugeuse. Anyone know an interpreter ?