In this trick, each right hand finger is one and the right thumb is five. Every finger on the left hand is 10 times bigger. Counting starts with a left fist and one right hand finger extended (one) and continues until four right hand fingers are extended (four) then the right thumb is the only thing extended (five). Next extend one right finger (6) and continue until all right hand fingers are again extended (nine). Then extend one left finger and make a right fist (10). Redo the right hand sequence until there are two left hand fingers extended (20). Continue until all the digits of both hands are extended (99).
To add or subtract, enter the first number then either add or subtract the second number. For example, to add 23 to 11 put up your fingers to form 23. This is done by extending two fingers on the left hand and three fingers on the right hand. Now add one more finger to each hand to get 34. To subtract 11 from 23 lower one finger on each hand to get 12. Sometimes this is a little more complicated. For example, if it is impossible to add a 4 you might have to add a thumb (5) and lower a finger. Other times you have to borrow or carry -- as in any addition or subtraction system.
Hold out your hands in front of you with all fingers extended. To multiply a digit by nine, count over from left to right until you reach that number and fold that finger down. Read off the digits to the left and right of the finger that is folded down. For example, to multiply 9x7, count over to the seventh finger and fold it down. To the left of the folded finger there are six fingers, and to the right of the folded finger there are three fingers. This is your answer: 63.
The knuckles, and the depressions between them, match the long months (31 days) and the shorter months (fewer than 31 days). Ignore the thumbs and use the index finger of one hand to count across the other fist. January is the left pinkie knuckle and February is the depression between the pinkie and ring finger. The ring knuckle is March. Continue across the left hand until July -- the knuckle of the index finger. August is the index knuckle of the right hand. Continue across the right hand until December -- The right ring knuckle.