Consider the herbalist's scales: things are carefully weighed out to achieve a desired balance. Magic plays with balance too. Sometimes the desire is for all forces to be equal and harmonious. At other times goals are accomplished by deliberately, consciously tipping the scales to provide the required effect.
Left and Right:
Once upon a time, not too long ago, the concept of Dualism associated the direction right with God, high, male, and all those good things. Thus children were forced to use their right hands, the dexterous hand, whether that was their dominant hand or not. Children who were naturally left-handed (the sinister hand) literally had it beaten out of them until they were dexterous too. Consider various phrases: in the right, Mr. Right, the left-hand path, a left-handed compliment.
Connections between genders and these directions, however, predate Dualism and Christianity. They exist in completely unrelated cultures, including some that were isolated for a very long time. Hawaiian magic, for instance, associates left with female, right with male as surely as the Chinese and many Asian, African and European traditions. Frequently offerings to female deities are made with the left hand (by both men and women) while offerings to males are made with the right (by both men and women). Feng shui suggests that it's beneficial for women to sleep on the left side of a common bed, while a man should sleep on the right. (Although spells intended to reverse the power dynamic within a relationship may suggest those sides be switched.)
Sometimes spells direct that an action be performed with either your left or right hand. Either one of two things is being requested.
In newer spell books, where the importance lies in not using your dominant hand, this is specified in spell-directions. Left-handed people will be directed to use their right hands. Where no such direction is given, if the only stipulation is to use your left hand, then this applies to everyone across the board.
Article from: Encyclopedia of 5,000 Spells