This article describes how to dispatch a job to overflow.
How to manually dispatch a job to overflow
While on the dispatch screen, click assign job, then select the overflow vehicle and click Assign job.
How to auto dispatch a job to overflow
On the auto dispatch policy that is attached to the site in question, add a rule that dispatches the job to the team that contains the overflow vehicle you wish to overflow the job to. (Note: To do this, the overflow vehicle should be set up on it's on sub-team, separate from any in-house drivers.