A solo drill that can be applied to BJJ, Judo, or other martial arts using kimura-like grips/submissions is to practice the movement on a sturdy piece of dowel or closet rod, broomstick, piece of pipe, exercise bar, etc. as a prop.
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Grip the object near one of the ends.
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With the other arm, overhook the object, swimming the arm into the kimura grip, gripping the opposite wrist.
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The bar should be under the armpit of the overhooked arm, granting the ability to apply pressure to the kimura.
Practice applying pressure, accuracy of the overhook and grip, switching sides, etc.