The flashbang can be used to clear a room and be nearly invincible while doing it.
You toss it in, and the AI is incapable of doing ANYTHING, and you rambo in and kill as many as possible before they recover.
Clearing a room can be done with 1 flashbang, and one person.
Some rooms have a door on both sides, so when you toss a flashbang, it might go all the way through the room and it might not do anything. To solve this, click in the center mouse button, or the mousewheel, and the player will roll the flashbang in, but be careful, because it might not go far enough and you'll flash yourself!

Throw your flashbang into the sky when the enemies aren't in buildings or in cover and look straight at the ground.

Get a shotgun and flashbangs and throw them at enemies anywhere and use a laser sight if possible to quickly dispose of them.

That was my guide on flashing.
I hope this will help you make use of the flashbang when playing Checkpoint or even Hunt!
I made this guide so people will not think the flashbang is a useless thing in game for realism.