A photonic shockwave is a method of disabling a small group of ships in a space battle. First known to be used at the Battle of Vorkado, a Romulan captain disabled two ships with this shockwave.
The knowledge of this battle was in the the Doctor's tactical database, as a part of his Emergency Command Hologram programming. He used this maneuver to disable two Quarren starships before retreating.
The effect is achieved by firing a photon torpedo at the enemy, then firing a phaser at the torpedo as is it between the two enemy ships. Exact timing is required for achieving maximal efficiency. Due to his computer controlled nature, the Doctor was able to pull off the move with high effectiveness. (VOY: "Workforce, Part II")