* Increased Surface Area: The rings on a ringed nail significantly increase the surface area in contact with the surrounding material (typically wood). More surface area translates to more friction.
* Mechanical Interlocking: As the ringed nail is hammered in, the wood fibers compress and then spring back into the grooves between the rings. This creates a mechanical interlock, where the wood actually grips the nail along its entire length. This interlocking effect greatly resists the nail from being pulled out.
* Finishing Nail Comparison: Finishing nails have smooth shanks. They rely primarily on the friction from the tight fit within the wood fibers to hold them in place. They don't have the mechanical advantage of the rings to prevent pull-out.
* Reduced Wood Splitting: While it might seem counterintuitive, some types of ringed nails can actually reduce wood splitting compared to smooth nails. The rings help to compress and hold the wood fibers together, preventing them from splitting apart as the nail is driven in. (This is highly dependent on the nail type and wood species.)
In summary, the rings on a ringed nail create a much stronger bond with the material it's used to fasten because of the dramatic increase in surface area and the mechanical interlocking effect.