A harmonic amplification effect is a dangerous spatial phenomenon encountered in a black cluster. Black clusters are known to contain many intense gravitational wavefronts that seemed to cause damage to starships within the cluster and naturally the crew of the ship would raise shields. However, the phenomenon causes the intensity of the wavefronts to increase proportionally to the strength of the ship's shields. In simple terms, increasing efforts to protect a ship from the wavefront makes them stronger but making no effort renders the wavefronts harmless.
The SS Vico was destroyed by a gravitational wavefront in 2368 when the crew of the ship continued to raise their shields against the wavefronts. The USS Enterprise-D was perilously close to being destroyed by the same effect a few days later until Data realized that there was a direct correlation between shield strength and gravitational wavefront intensity. When the Enterprise dropped her shields at the last minute, the wavefront passed over the ship without causing any damage. (TNG: "Hero Worship")