Katsuki condenses his explosions, so instead of letting off a singular explosion, he lets off a barrage of much stronger explosions, which can be applied to his existing Ultimate Moves as a form of upgrade. He describes the attack as being one he can do when he feels he's in a sensation of total crisis. He later figures out the best approach to execute the technique is by storing up a bunch of sweat so he can create the barrage of explosions rather than just one big one.
Cluster is first used inadvertently, in the form of a mobility move, by Katsuki to defend Izuku Midoriya from Tomura Shigaraki during the Paranormal Liberation War. Also during the War, Cluster is used again in the form of the same mobility Ultimate Move demonstrated before to help Best Jeanist. Both Cluster and the mobility move get their names, with the move being named "Blasting Speed Turbo: Cluster," when they are used in an effort to bring Izuku back to U.A.
The second application of Cluster is used in the form of Howitzer Impact: Cluster. This time, Cluster is used offensively and in combination with the Strafe Panzer support item to provide a more powerful attack, as well as a spray of explosions in multiple directions.
It is later revealed that another, more dangerous application of Cluster can be utilized. By using his Cluster techniques continuously without rest, Katsuki puts tremendous stress on the sweat glands on his palms, causing the stored up sweat to be transferred to other openings of the body, essentially granting him the ability to execute Cluster all over his body, giving him a tremendous boost in speed and power that can briefly rival "Tomura", at the cost of putting further strain of his already damaged body.
- Cluster munition - Wikipedia article about the type of munition this Ultimate Move is named after.