An old carpenter’s trick to keep nails from bending when they are pounded into hard materials is to grip the center of the nail firmly with pliers. Why does this help?

Two handles on a small tool for pulling, grasping, or cutting are pressed together to merge two specifically formed pieces of metal at the other end.

Gripping the center of nail firmly with pliers, increase the moment of inertia. Now, it requires greater amount of force to bend the nail.

Normally when force is applied on the nail, the nail may bend from the straight position. This bending will cause the shear stress on itself. When a plier is employed, the chance of bending is reduced, lowering the risk of shear stress. The nail will remain mostly straight as it enters the hard material.

Moreover, when carpenter holds the nail with pliers, it increases the moment of inertia of system (nail and pliers). Therefore, any bending of the nails will withstand more rigidity. Therefore, holding the pliers provides a precise hole and prevents bending.

Further, gripping of the nail with the pliers increases the surface area. That’s why applied force gets distributed to a larger area. it prevents bending of nails.

$∆x=\frac{\text{F}}{\text{SA}}\text{L}$

It is clear from the formula that shear deformation is inversely proportional to the surface area. So, if area increases there will be less deformation.

Therefore holding the nail with pliers increase the surface area that’s why nail remains straight while getting into the hard material.