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.
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.