Slave branches are used to thicken a bonsai trunk or even branch. By allowing wild and untrimmed growth of these branches the trunk or branch can be thickened.
Once the thickening is done the slave branch can be removed or trimmed back.
Wild and untrimmed growth to thicken this branch on a Ficus natalensis/thonningii complex bonsai
The branch has been shortened back once it has the proper thickness but needs secondary and tertiary branches to be developed
The branch thus thickened, as in this case, now has a proper thickness but will need more work to ramify it and develop secondary and tertiary branches. This can be done with repeated nipping out of buds and defoliation techniques.
Aerial roots are kept on this branch as they speed branch thickening
On this bonsai Ficus natalensis the lowest branch on the right side is not thick enough. Most often the lowest branch should be the thickest on the tree. In order to achieve proper thickness the branch is allowed to grow longer than the style dictates. Note the leaves are also larger than the rest of the tree which is being trimmed regularly.
In a year or two the branch will thicken because of its continued growth and then it will be appropriately shortened. The next step will be to continue the process of shaping the branch properly.
The area in red is allowed to grow without trimming to thicken the branch