Arm bars from bottom - GET HEAVY ON THE HEAD
Arm bars (juji gatame) are among the most versatile weapons in Jiu Jitsu - being effective gi and no gi, standing or ground, top or bottom position. When performing them from underneath the main problems you run into will always be stacking and posturing up and away. Both reactions, if done well, can subvert your ability to finish your arm bar. Both reactions start with head position. It is crucial therefore, that you get HEAVY ON YOUR OPPONENTS HEAD WITH YOUR LEGS in order to prevent forward head movement (stacking) or up and away head movement (postural escapes). Look at Nicky Ryan finishing a beautiful juji gatame arm bar variation that involves two legs covering the head for extra weight over the head and a very strong finish. Next time you work juji gatame from bottom position really emphasize getting heavy on your opponents head as he begins to resist and watch your finishing percentages rise with this classic Jiu Jitsu finishing hold.