This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
A nail pulled with an ordinary claw hammer will be bent in the operation, and for this reason the double claw is used to draw the nail straight out of the wood. An ordinary claw hammer can be easily converted into a double-claw by filing out one of the claws as shown. The notch is filed only large enough to slip under the head of an average-size nail. After drawing the nail a short distance In the usual manner the small notch is set under the head of the nail which is then pulled out straight. - Contributed by J. V. Loeffler, Evansville, Ind.
Ill: The Small Notch on the End of the Claw Makes It Easy to Pull a Nail Straight