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Space between the hands allows a larger initial kink.
Relying largely upon wrist strength (radial deviation) to transfer force from the upper body through the bar, double underhand bending is performed by grasping the bar at chest level with an underhand grip. The pinkies are then driven down through the center of the bar, bending the ends up. Most of the strength for the bend comes from the triceps and upper back. Crush the bar as hard as possible with the hands to effectively transfer force into the ends of the bending stock.