Glossary /// Alias
Arm-in rear triangle escape
Also known as Back Triangle Escape — the canonical term used on this site.
Descriptive — rear triangle with one arm inside
Arm-in rear triangle escape is the descriptive name for back triangle defence, distinguishing the version with an arm trapped inside the legs from the arm-out rear triangle that finishes faster.
Etymology. “Arm-in” is the qualifier that decides everything. With a shoulder inside the triangle the strangle is slower and the defender has a real window; with the arm out there is nothing between the legs and the neck. The rear qualifier marks the entry as back control rather than guard.
Mechanics. The window is the leg swap: extracting the trapped arm while the attacker rearranges their legs is the highest-percentage moment, because the lock is momentarily incomplete. After that the defence is to keep the position moving and, late, to stack the lock so the angle never closes.
Cross-reference. “Arm-out rear triangle escape” is the faster sibling. Full escape coverage on Back Triangle Escape.