What is 'double-declutching' (double-clutching), and in what situation might it be used in light vehicle driving?

1. An old shifting technique where the clutch is depressed twice to match gear speeds, used when synchromesh is faulty
2. Pressing the clutch and brake pedals simultaneously in an emergency stop
3. Engaging the clutch fully while the vehicle is parked to lock the transmission
4. Pressing the clutch pedal halfway to slip the clutch on a hill