Ashi-Hishigi also called an Achilles lock or simply an ankle lock, is a technique described in both The Canon Of Judo[1] by Kyuzo Mifune and Brazilian Jiu-Jitsu, Theory and Technique[2] by Renzo Gracie and Royler Gracie as well as demonstrated in the video, The Essence Of Judo.
It is classified as a joint lock (Kansetsu-waza), and is not a recognized technique of the Kodokan.
However, it is a commonly used technique in both Brazilian Jiu-Jitsu tournaments and Mixed martial arts competition.
Executing this lock involves pressing the achilles tendon into the back of the ankle or lower leg.
It is typically performed by wedging a forearm, especially a bony part of it, into the Achilles tendon.