From the American Immigration Council:
This Practice Advisory addresses when the voluntary departure period runs and the events that cause automatic termination of a voluntary departure order; the serious consequences that result from failing to depart; and when these consequences do not apply.
The Advisory also analyzes important changes in the case law and outlines arguments for when the consequences for failing to depart pursuant to a voluntary departure order should not apply.
The Appendix provides background about the eligibility requirements for voluntary departure.
You can view the document here.