We can accept that something is unrealistic, as long as it's very cool.—TVTropes' Laconic entry for the Rule of Cool.
The Rule of Awesome (or Rule of Cool) establishes that some rules (natural laws, narrative rules, plot, etc) are secondary, invalid, forgotten and/or abandoned if a particular event is awesome enough.
This, of course, applies to works of fiction. IRL, awesomeness can't invalidate scientific laws (although there are some awesome things in Nature).
It's important to notice that this rule is in effect when there's no reasonable explanation or justification for the awesome event. If there is a reasonable explanation for an awesome event to happen over a non-awesome one, it's not an example of the Rule of Awesome:
...after all, the entire point is that it's about things that don't make sense but we still gladly accept just because they're so cool
- MOAR DAKKA. Firing one weapon in each hand is extremely dangerous, not to mention inaccurate and wasteful. But it looks awesome
- Star Wars' lightsabers. An energy based weapon, able to cut flawlessly through most materials at any angle. Has no weight (besides its hilt) and you have to train for years starting in your early childhood to be mildly proficient at it, provided you're even somehow attuned to The Force. Still, it's awesome.
- There's no plausible explanation to why the ground cracks up and green fumes come out of the fissures during Be Prepared in The Lion King
- The Adventures of Dr. McNinja. Just look at the title, for Chrissake
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