What does the 'wobble' or 'tank-slapper' mean and how should you react?

1. It is a normal startup vibration; ignore it
2. The front end shakes violently; close the throttle gradually, grip the bars firmly but not stiffly, and avoid braking until it settles
3. Pull the clutch in and brake hard to stop it
4. Lean sharply opposite the direction of the wobble