lasting technique of hammering set lasting tacks down and toward the midline of the last, bending the tops of their shanks toward the midline
Bending tacks serves a few purposes:
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Slightly increase tension on the material.
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Prevent the lasted material from riding up the shank of the tack and loosening.
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Move the heads of set tacks out of the way of later tacks, creating more space above the insole close to the featherline.
However, bent tacks have to be straightened before using again. More often, perhaps, bent tacks are simply discarded.