Flashy moves involve moves and/or combos that command respect in that they are for show rather than for efficacy, usefulness, etc.; by nature, they are difficult to use.
In the case of moves, this can happen because they aren’t useful on their own and don’t go in combos well if at all, or they may not have any useful properties in particular.
Some moves, like the one-hit-kill moves in Guilty Gear count because there are few setups for them; the equivalent in Hokuto no Ken does not, because there are actual ways to use them.
Basically, use them because you’re being a bit romantic (or Globetrotterish), not because they’re useful. They are the opposite of useful.