When Renzo Gracie talks, I listen. There's very little the man doesn't know about hand-to-hand combat, specifically when it comes to groundwork and takedowns. In this case, he goes into an examination of an arm-lock.
Renzo Gracie displays an arm-lock transition from the over under turtle position and explains how he does it to a group of kids at Codella Brazilian Jiu-Jitsu Academy.
Take a look:
In the turtle position, he says, a lot of people make mistakes and put themselves at risk
He says maintaining your leverage is monumental...
This is where you don't want to go...
It's all about sliding your knee to your hand... pretending you want to get up and actually putting pressure on the guy on the ground
And here is the final result