Voluntary Overtime is the practice of providing developers with meaningful work and with other contributors to internal motivation so that they will want to work more than they're required to. The extra hours provide a direct productivity boost, and the extra motivation gives developers an edge that transfers to their regular work hours. Moderate, voluntary overtime can be used in virtually any environment, but its applicability is limited by the fact that both it and excessive, mandatory overtime are already in widespread use.
Potential reduction from nominal schedule:
Improvement in progress visibility: ...