The following technique of bending nails was developed by oldtimer Peter Lindop - have fun!
How to bend nails with your hands?
According to Mr. Lindop Nail Breaking is one of the best ways of developing one'e forearm, and to demonstrate one's forearm and wrist strength. The man in the street or at your place of work may not understand the strength that is involved in lifting a heavy weight, or pulling a heavy set of cables (strands), but almost everybody understands the strength of a six inch nail which you can carry around in your pocket. Most weight trainers can bend nails, but you can count on the one hand the number who can break one in less than thirty seconds; many have the strength, but few have the 'know-how'.
You must have rather strong wrists and forearms and, therefore, he suggests at least six months training under your belt before trying 'Nail Breaking'.
The first requisite is a piece of thin canvas, 12 x 8 inches would be the ideal size. In one he read, the author advocated a wash leather, but he finds canvas to be far superior.
You must know wrap the nail lengthwise in the canvas as tightly as possible. We then hold the nail in front of the knees. This is to get the first bend in the nail. Remember to keep the feet in a good solid position and pull the outer ends of the nail towards the body. To further bend the nail, we use the inner part of the thigh, and bring both thighs together.
There is also a second method for those with strong arms and shoulders and many prefer this second method. He used to keep his hand in and used both ways. Do not bend the nail until both ends meet or it will be difficult to open again.
You can also wedge the right hand on the outer part of the left hand until the nail is almost straight. Then do like before and press in again, use each position alternatively until the nail breaks (usually six times in each position). When you get going, and used each position, you can speed up the action, and the increased friction helps the nail to break more quickly.
He finds it better to attack the nail as you would when cleaning a heavy weight. You will find that if you perseve with the above methods, it won't be long before you are breaking with ease heavy gauge sic inch nails.