The americana (ude garami in judo terminology) is one of the most fundamental shoulder locks in BJJ. Applied from mount or side control, it attacks the shoulder's internal rotation limit using a figure-4 grip.
The americana grip is a figure-4: control their wrist with one hand (palm down), pass your other arm under their elbow, and grab your own wrist. This creates a powerful double-arm lever.
From high mount, isolate one of their arms by pressing your knee against their elbow. Apply the figure-4 grip, then rotate their arm in a windshield-wiper motion toward the mat.
From side control, isolate their near arm by controlling the wrist and pressing their elbow to the mat. Apply the same figure-4 grip and execute the rotation from a perpendicular body angle.
When they grab their own leg, apply downward pressure on their elbow while maintaining rotation. The combined torque overcomes the grip defense.
The americana attacks internal shoulder rotation; the kimura attacks external. They are mirror images of the same figure-4 control, attacking in opposite directions.
See also: Kimura System, Omoplata System, Shoulder Lock Guide, Mount Control