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Begin with the feet under the hips, the back set, weight evenly on the feet, dumbbells at arms' length and held close to the legs. The movement can be started with the dumbbells at mid-shin height for a full range of motion, but starting at around knee height is recommended.
Extend the legs and hips powerfully, shrugging the shoulders as they finish extension, and swinging the elbows under the dumbbells rapidly as you pull your body down into a squat to catch the dumbbells on the shoulders, keeping the dumbbells close to the body. This is not a jumping curl: the dumbbells travel upward by the force of the legs and hips, not the arms.