"50% chance for Confused and/or Blinded against one enemy. It works in a way that only Isabelle can do."
Cost: 5 TP
Source: Ganguro Isabelle - Starting skill.
Flash is a skill that with a 50% chance each to inflict the 
Blinded and 
Confused status effects on one random enemy.
This skill actually involves Isabelle flashing her breasts at a random enemy. Eric(a) will complain about Isabelle's "methods" of attack the first time, and then again every few battles for a total of four sets of unique dialogue after which it will stop. These occur at the first, fifth, tenth, and fifteenth uses.
Though it might be overlooked in favour of more directly damaging attacks, note that the Shining Dawn Bird and Starheart are both susceptible to blindness, especially the latter, which can trivialise the battle if it connects.