Expansion Anchor Bolt Features a round slot head. Working principle of expansion screw: Put the expansion screw into hollow wall and tighten the nut of the expansion bolt with a wrench. Bolts out while the metal sheath doesnt move. Therefore, the large head of the bolt expands the metal sheath to fill the entire hole. Expansion screws cannot be pulled out. Mounts to a hollow wall of drywall to hold shelves, curtain rods, hole stands and ramps