Slave branches are used to thicken a part of the trunk or branch that needs to be heavier. Slave branches are allowed to grow wildly without trimming and when the thickening has been achieved they are removed.

This Ficus natalensis has a slim trunk that would be improved by thickening it to introduce some more taper. The branch outlined in red was allowed to grow for a year or two to thicken the trunk.

Slave branch on this Ficus natalensis to thicken the lower trunk of the tree

Lower trunk has been thickened sufficiently so the slave branch has been removed