Leaf Pumping is a technique which allows you to maintain height while gliding with the deku leaf--making it possible to reach ledges that would otherwise be out of reach. It is important not to confuse leaf pumping with the "leaf attack", which is simply pressing the button the deku leaf is on to release a burst of air and repel enemies such as peahats.
Leaf pumping is done by quickly pulling out and putting away the deku leaf repeatedly. It is important to switch between the button the leaf is equipped on and the A button very quickly otherwise you may lose height instead of maintaining it. Depending on where you are performing this trick, the speed and accuracy at which you must alternate may be either very demanding (as in the method of skipping the Earth Temple boss key below), or quite forgiving (as in skipping the flower surfing in the Forbidden Woods). Note that although leaf pumping does allow for a maintenance of height, it does not increase you forward momentum. It also drains your magic power much quicker than gliding normally, due to the fact that each time you pull out the leaf it uses a 1/4 of a unit of magic. (Double magic meter contains 8 units of magic; Normal meter contains 4.)