Phase shielding is a defense mechanism of capital ships.
The existence of a shield can appear as a flicker shimmering around a ship. Weapons display screens also show, along of a target, the pulse of its shielding.
A shield "breaks" when strong and repeated hits soak off all the energy from the ship's generators. According to the doctrine tought at the Confederation Service Academy, up to the Kilrathi crisis, only big battlewagons had the capacity to do it. In theory a big amount of starfighters hitting one single point of a capital ship could also do that, if it weren't for the ship's heavy antispacecraft guns.
Because shielding depends on the amount of energy generated from a ship's reactors (limited only by the area that the jump containmnet field can be generated by the jump engine generators), bigger ships could generate stronger shields, creating a theoretical paradiegm of balance between mass and tactical power. In practice, a 50,000-ton battlewagon had enough energy both to hold its shields and use its guns to fend off attackers.
Around the time of the Kilrathi crisis, technicians experimented with the idea (and possibly developed some prototypes) of a warhead that can punch through shielding; but simply increasing the frequency of polarity shift of the shield, would trick the warhead to blow outside. The House of Lord Vakka, under the orders of Crown Prince Gilkarg nar Kiranka, developed a shield-penetrating torpedo, capable of not detonating outside of a shield, but penetrating it, and damaging the ships themselves.