The transphasic torpedo is a weapon used by Starfleet and designed sometime in the 2380s in an alternate future timeline. It was developed to specifically handle the Borg threat. A single transphasic torpedo is capable of destroying a Borg Cube. This technology was introduced to Starfleet in 2377 by Captain Kathryn Janeway and the crew of the USS Voyager after returning from the Delta Quadrant with the help of an alternate future timeline version of Captain Janeway.
In 2381, the USS Enterprise-E was the only vessel that had been fitted with transphasic torpedoes. During the Borg sieges in February of this year, Captain Jean-Luc Picard recommended that the whole fleet be outfitted with the devices, to give the fleet a better chance of survival. He was over-ruled by Admiral Alynna Nechayev, who feared that the more the weapons were used, the more opportunity it gave the Borg to adapt. Once the Borg invasion began, the Enterprise and Starfleet Command distributed the torpedo's specifications to every vessel they could. Starfleet also provided the technology to the Klingon Defense Force. By the conclusion of the Borg conflict, the Borg ships had adapted to transphasic torpedoes before the invasion could be stopped.
In 2383, the torpedoes were stored in Starfleet's experimental weapons headquarters. It was noted by engineers that the transphasic technology used in the torpedoes could be adapted to other parts of vessels' tactical systems. Using the principles in shielding caused a power drain, cutting warp speed (down to 9.1 on Sovereign-class starships), had the potential to quickly burn out shield emitters, and the Borg would be able to utilize wide-dispersal firing patterns to counter. The same principles could not be applied to phaser systems without an iron-60 crystal matrix to handle the strain, but it wouldn't output at sufficiently high energy levels to be effective.
The technology is based on producing a very destructive subspace compression pulse. The torpedo, upon detonation, delivers the pulse in an asymmetric superposition of multiple phase states. Borg shields can only block one subcomponent of the pulse. The other subcomponents go through the shields and deliver the most of the pulse to its intended target. Every torpedo has a different transphasic configuration, randomly generated by a dissonant feedback effect to prevent the Borg from accurately predicting what the configuration of the phase states is.
In Star Trek: Novus transphasic torpedoes caused molecular structures to gain and lose energy which thus, very quickly, causes a loss of cohesion. It takes around 1 minute for full loss of cohesion to take a toll.