The armbar (juji gatame in Japanese) hyperextends the elbow joint by trapping the arm between your legs, controlling the wrist, and lifting your hips. It is one of the highest-percentage submissions in BJJ and is available from almost every position: closed guard, mount, back control, and many scramble positions.
The key mechanics are the same regardless of position: isolate the arm, control the wrist with both hands (thumbs pointing toward their fingers), position your hips high near their shoulder, squeeze your knees together, and extend your hips upward. Applying pressure slowly and controlled is both safe and more effective — a sudden yank gives the opponent time to react with a counter or stack.
The armbar is usually one of the first submissions beginners learn because it appears early in the closed guard curriculum and teaches important hip-movement concepts that apply elsewhere.
For the full closed-guard version with step-by-step instructions, see Armbar from Closed Guard.