The americana (ude garami or keylock) is a shoulder lock applied from side control or mount. You pin the opponent's bent arm to the mat and use a figure-four grip on their wrist to rotate the forearm upward and outward, stressing the shoulder joint in internal rotation.
The setup is straightforward: isolate the near arm, bring their wrist to the mat, thread your arm under their wrist, grab your own wrist from above, and slowly move their forearm toward their head. The pressure is gentle to complete — applying it slowly gives your partner time to recognise and tap.
The americana and kimura attack the same shoulder joint but in opposite rotational directions — ude garami (internal rotation from below) versus reverse ude garami (external rotation from above). Both are foundational submissions taught early in the BJJ curriculum because of their clear mechanics and the ease with which beginners can feel the lock.