The S-Mount Armbar looks a lot like my previous video the Lazy Armbar but there are a few key differences..
Notice that I start off by placing my fists on the floor and then taking up all the space between my thighs and my opponents shoulders. This will insure a snug fit between your thigh and your opponents triceps.
Once you have taken up this space it is important to lay your chest on the back of your opponents arm, this will be uncomfortable for your opponent and make it difficult for your opponent to escape when make the S-Mount transition with your legs.
When I make the transition into S-Mount I lead with my arm and control the elbow first and then bring my knee to my opponents ear. If you rotate first it should make getting your ankle under the shoulder easier. Make sure to get that ankle all the way under the shoulder and create a squeezing pressure between your knee by the ear and the calf under your opponents arm. Please reference the video to see the angle of the ankle and the body pressure.