Zealot bombing is a technique used to destroy Siege Tanks with their own splash damage. Most of the time this is achieved by using a Shuttle to drop a low number of Zealots right on top of the Tanks. Alternatively, other units such as Dark Templars, Dragoons or Probes might be used.
Ideally, Zealots should be dropped as close to the Tanks in Siege mode. Additionally, Zealots can be sacrificed to defuse (Mine Dragging) a larger mine field. Instead of manually dropping units one by one on top of a longer Tank line the Move-Drop technique can be used to spread Zealots equally with less mechanical input.
Dark Templars are an alternative, even though this move is more expensive. The trade off between costs and results should be added before suiciding the units. The cloaked units are especially effective, when no Turrets or Science Vessels are brought into the fight. Only Comsat Sweeps (Scans) can be used to detect these units. However, if a Dark Templar is picked up shortly after a Scan went down, the unit will remain visible for a short time frame after unloading again, regardless of missing detectors.
Dragoons and Probes are mostly used to stop an attack in the last minute, mostly during the early to mid game. The disadvantage of Dragoons is their size, they will add less splash damage to the surrounding Tanks and Vultures. Probes have a similar problem, as they can not be used to trigger mines, even though they are mostly less costly.
When executed correctly, Zealot bombing can make breaking a Terran push, especially an early game push, much easier. The incoming forces can be either be lowered, or the incoming attack can be slowed down. As secondary objective, fire can be drawn on the less important units like Zealots, while the rest of the ground force can move in without taking too much damage. Furthermore, the Terran player is forced to either add Turrets or Goliaths to his army composition to be completely safe against this technique.