Limit Break allows you to promote your low-level fighters without spending resources.
Fighters that are placed in the slots are promoted to the lowest level of the Monument Fighters. You can remove a fighter from the slot by clicking on it, but once you do so the slot enters a 22-hour cooldown. After the cooldown ends, you can slot in another fighter.
Please refer to the Limit Break rules by accessing ℹ️ in the left upper corner.