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The Heel Hook is a LegLock submission. This technique applies pressure to the knee joint of an opponent, and when properly applied will force your opponent to submit or destroy the ligaments in their knee.

To defend the Heel Hook, you need to remember to "go with the flow", or the direction your opponent is twisting your knee. Do not go against the twisting, or that could damage your knee. Ideally, as you feel your opponent locking your heel, you twist your body in the same direction that your opponent is twisting the submission, but you twist very fast to "outrun" your opponent. 

To complete your escape, you must slide your knee (which is affected by the heel hook)  to touch the ground. Once this happens, you will be free of the heel hook. Continue spinning until your knee is free, but don't be afraid to tap out. It's better to tap out than risk knee surgery later (ligament tears from heel hooks and kneebars do not heal on their own and require surgery).

The following video is a good example of the escape from the standard Heel Hook:

Note that the direction your escape twists will differ if your opponent applies a reverse Heel Hook on the same foot. Be sure to drill these movements lightly at first with your training partner, and practice this escape often.

If you are a beginner to leglocks, work with an instructor or other experienced grappler. LegLock submissions such as the Kneebar and Heel Hook are more dangerous than the armbar, because the onset of pain occurs when damage occurs, whereas with an armbar pain usually occurs before any serious lasting damage. So you should always exercise caution against beginners or others who may not want to tap.

Heel Hook demonstrated by Jess Laudin:

Note in this video that Jess uses this right arm to cradle his opponents left foot. This differentiates the Heel Hook from the "Reverse" (aka Inverted) Heel Hook, where Jess would cradle either the opponents left foot with his left arm, or cradle the opponents right foot with his right arm. To escape, his opponent should rotate his left leg in a "clockwise" direction  until he can get his left knee free.

Bas Rutten Demonstrates the "Reverse" Heel Hook:

Note again the placement of the foot, to indicate that this is not the standard Heel Hook. To escape the hold, the defender should rotate his left leg in a "counter-clockwise" direction and free his left knee. 

Heel Hook Takedown with Masazaki Imanari:

This setup is advanced and flashy looking, but can be effective with practice. Note that Masazaki ends in a "Reverse" Heel Hook position.

Students practicing Sambo Leglock attacks:

This may give you more ideas of the multiple ways these leglock attacks can be setup.