It seems like the Seraphim (and sometimes King) have their flames go off only when they are attacking. Some examples of this are S-Hawk trying to attack Luffy’s group with Daz Bones’ devil fruit, S-Shark trying to attack Nami, and King anytime he uses Tempura Udon.
We saw this previously also when we first met S-Shark and he attacked Nami, his flames went off then too and basically anytime he was charging in to go attack. Interestingly though when the seraphim shoot out projectiles that don’t need physical strength to use, the flame on their backs remain on, always e.g shooting lasers.
The question most fans have been wondering though is, why even go into speed mode? Why does every Lunarian always go out of defence mode?
My theory is that Lunarians only have a default mode (defence) and a offense mode (flame off). In their default state with the flame on their backs, Lunarians are super durable as we know, but when they switch to their offense mode their offensive abilities all get a huge buff or you could also say in defense mode their offensive abilities are nerfed, either way in order to attack at their full power they need to turn the flames off.
The reason this happens is because switching to offense mode builds up some explosive energy inside the Lunarians, which the Lunarians can convert into a burst of speed or power.
Due to needing to switch to offense mode in order to create this type of energy, that is why King didn’t just overwhelm Zoro with speed in their fight, King’s speed is more of a burst than him full on becoming Barry Allen.
The Lunarian can also decide to not use this built up power through increasing their speed or strength, and instead go back into defence mode and just use that energy to blow themselves up, as experienced by Zoro:
Default mode allows them to survive this explosion, due to their high defence, and they additionally do major damage to any enemies near by.
The flame on the backs of the Lunarians is some kind of special fire, it might even be the fire Lilith was talking about with an absurdly amount of energy and when it’s outside the Lunarians’ body, it strengthens the power of their skin to crazy levels which explains why attacks on their skin have no effect but this means internally damaging attacks would, like potentially Zoro’s conquerors haki infused attacks.
And when the fire gets absorbed inside the Lunarians’ body, their skin is no longer empowered but they gain the explosive energy that can be used to make a more powerful attack, move faster or just release the energy as is causing the Lunarians to explode.
Theory by Legitimate_Cow7198 (https://www.reddit.com/user/Legitimate_Cow7198)