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Normally, fire, as I understand it, is a relatively low-density plasma (thus, while theoretically capable of reaching temperatures of thousands of degrees, is incapable of transferring much of that heat to other objects). The so-called "laser sword" appears to be suspended plasma with a higher density, thus being capable of transferring more heat to objects, and therefore able to shear through whatever metal the robots are made of (as evidenced by the glowing yellow particle effects when your sword connects). Presumably, then, the "fire" upgrade dramatically increases your sword's ability to transfer heat (by further heating the plasma, increasing its density, or some combination thereof), to the point where enough heat is transferred upon contact to actually melt the metal (rather than heating it to the point of plasticity, as above), causing more widespread damage.