I can imagine the Adventurers doing a small jump or an acrobatic move inside the fireball, But the adventurers remain inside the fireball!
And if they succeed, they should not be relocated of their position?
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There isn’t one, at least not one that bears close scrutiny. Like HP itself, explaining Reflex saves—particularly in the face of evasion that allow one to take no damage on a Reflex save—just doesn’t really work out.
Various partial responses can be made; you can argue that you turned to show a slimmer profile, temporarily took advantage of some cover, or what have you, but ultimately someone in a sealed, featureless room can use Reflex to avoid damage from an explosion that fills that room utterly. There is no way to explain that,1 but that’s how the game works.
And it works this way because simplifications are necessary to make the game playable, and trying to make this more “realistic” results in extremely complicated gameplay that just isn’t worthwhile for most games of D&D. By complicating these matters, you don’t just enhance realism—you also demand more time and attention paid to these corner cases, which means you are taking time and attention away from other things. The designers of D&D decided that they didn’t want to divert time and attention in these ways, because they wanted to focus more on epic narratives and daring heroics, with perhaps just a pinch of impossibility thrown in for fun.
And really, it gets very difficult to “fix” this. There are always trade-offs. That being the case, just accepting the limitations of the system in favor having a smoother, faster game is a choice that I, personally, strongly agree with.
It’s also important to remember who and what D&D adventurers are: they are superhuman by definition. They engage in mighty feats well beyond the capability of real-life humans on a regular basis—and must do so to survive, much less thrive. Evasion is an extraordinary ability—that is, nonmagical. How? Because what’s “extraordinary” in D&D may very well be actually impossible. The very definition of “extraordinary” in the game explicitly tells us this:
Extraordinary abilities are nonmagical, though they may break the laws of physics.
It’s important, when playing any game, to remember what that game is for and what kind of narratives it supports. This kind of ability is part-and-parcel with the kind of game D&D is trying to be.
This can be done through various methods such as interposing objects between the player and the effect such as a cloak, a thin piece of cover, partial cover, etc. It can also be done as a reflexive dive for said cover. Or it could be role played as simply diving below the effective blast, but not out of it in it's entirety. Or it could be simply reducing the amount of your body hit by the spell like the forbidden school of monk training known as the hidden iron turtle (the fetal position, I'm talking about curling up in the fetal position).
Ultimately, the role play is up to the players and the DM.