A slightly more advanced technique to propagate Bonsai is air-layering. The principal of layering is to force a tree or branch to form new roots at a certain point by interrupting the stream of nutrients from the existing root system. This means you can use air-layering for several purposes; reducing the length of a trunk, growing a better Nebari (root flare or surface roots) or selecting a branch to be grown as a separate tree.
Air-layering should always be done during the spring, when the tree already started growing after its winter rest.
There are two main techniques to air-layer a tree; the tourniquet method and the ring method.
The tourniquet method involves tightly wrapping the trunk/branch with copper wire to block the stream of nutrients partially. When the trunk/branch grows thicker the stream of nutrients will decrease more and more, forcing it to grow new roots just above the wire. This method is used for rather slow growing trees that need more time to grow new roots; these will not survive the more aggressive ring method.
The ring method involves cutting away a ring of bark at the point on the trunk/branch where you would like new roots to grow. The portion above the ring will have to grow roots immediately in order to survive. The ring should be wide enough to prevent the tree from bridging the gap.
The moss should be kept moist at all times. After about one to three months roots should be growing in the moss. When the bag is filled with new roots carefully cut the layer just underneath the new roots. Do not try to remove the moss or sort the roots; simply plant the entire bundle without disturbing it in akadama, fine gravel and potting compost mixed together in a ratio of ½ to ¼ to ¼. Keep the tree protected from low temperatures and wind; a greenhouse or cold frame can be very useful. Leave the tree untouched until the next spring, when it can be trained for the first time. Small quantities of fertilizer can be used during the first summer.