It is like moving from a whisper straight into a blast: the picture changes abruptly to a shot with very different energy or brightness. The contrast creates the surprise or joke.
Choose two shots with a strong contrast and place them directly against each other. Cutting the sound or starting a new sound on the edit can make the change hit harder.
A smash cut is still a hard cut, not an added transition effect. Use strong contrasts sparingly and let the first shot set up the difference.
When to use
Use it for a reveal, comic timing, or a sharp change in mood. Since contrast is the point, shape the silence or sound immediately before the cut too.