Glossary
Flash
In one line: An ability that briefly blinds whoever looks at it, used to enter or win a duel.
What it means
A flash is an ability that blinds you (fully or partly) for a moment if you are looking at it when it pops. Many agents have one, in different forms: thrown, bouncing, going through walls.
A flash on its own does nothing: it exists for someone who goes in right after. The right timing is "flash and go": the entering player must be ready to use the second in which the enemy cannot see. A flash that blinds your own team or pops where nobody is looking is wasted. To dodge one, turn away or stay behind cover when you hear it coming.
An example
A defender is holding the corner of the site. Your initiator throws a flash over the wall and calls "flash, go": the duelist swings while the defender is blind and kills them with no risk.