When driving a manual light vehicle, what is 'rev-matching' during a downshift, and why is it performed?

1. Blipping the throttle while the clutch is depressed to raise engine RPM to match transmission speed, preventing drivetrain shock
2. Revving the engine to its limit to clear carbon deposits from the spark plugs
3. Pressing the accelerator while the brakes are applied to shorten stopping distance
4. Keeping the engine idling at high RPM while the vehicle is parked