A strong-force reversal field was a type of field where the strong force that bound atomic nuclei together was reversed so that the atoms were repelled away from each other instead of attracting one another.
Species 8472 had developed a strong-force reversal field which could be regenerated by nine Species 8472 battleships forming around a Species 8472 frigate and directing the reversal field at a planetary body. During the Borg-Species 8472 War of 2373/2374, Species 8472 used the beam to destroy numerous Borg worlds. In January 2374, the crew of the USS Voyager witnessed the destruction of three Borg-occupied worlds first-hand and barely escaped the planetary debris after being tractored away by a Borg cube. (VOY episodes: "Scorpion", "Scorpion, Part II")