The ultra-power blast was a modification that allowed laser cannons to release their entire energy stores in one shot, thus creating a boosted energy bolt. It was installed by the Rebel Alliance on the modified TIE/LN starfighters of Flying Bantha Squadron.
The ultra-power blast was capable of penetrating even the most powerful shields and crippling Star Destroyers considerably.
The effect of an ultra-power blast on an Imperial-class Star Destroyer
In 3 ABY, the Rebel Alliance altered the laser cannons on the TIE/LN starfighters of Flying Bantha Squadron so that they could make use of an ultra-power blast. The TIE fighters were modified to fire only six laser bursts at standard strength, after which they would release their entire energy stores in a shot powerful enough to cut through even the strongest shields.
During their attack on the secret armada, members of Flying Bantha Squadron used their ultra-power blasts to severely damage Imperial-class Star Destroyers, thus creating confusion among the Imperial fleet. The officers of the damaged vessels followed standard procedure and counterattacked, but since it was unclear which ships had hit the targets, the warships lashed out at random, hitting many TIE fighters and even other capital ships, causing an escalation of confusion that spread throughout the fleet.
Luke Skywalker then used his ultra-power blast on Admiral Giel's Praetor Mark II-class battlecruiser, the Helmsman. The boosted energy bolt destroyed the transport cage containing the Teezl and created an explosive chain reaction which took a great deal of the flagship with it.