If a bar wants to escape your hand, it has two main ways it can do it. 1, it can slide out, Or 2, it can roll out. Both of these involve your fingers uncurling but rolling will tend to happen first. A cross grip means that the direction the bar has to rotate to roll out of one hand is the opposite of the direction to escape the other hand. The bar can’t rotate in both directions at once so it stays put. Now the only way you can drop the bar is for your fingers to totally uncurl and have it slip against friction on your fingers.
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