A standard left-click flashbang thrown high into the air is useless in higher Elo. The enemy will see it, turn away, and instantly snap back to kill you. You must throw "Pop Flashes."
What is a Pop Flash? A pop flash detonates the exact millisecond it enters the enemy's field of view. There is no visual warning and no audio cue of it bouncing.
How to execute them:
The Geometry Bounce: Throw the flash backward against a wall or the floor just before a corner. The grenade absorbs the travel time out of sight and pops exactly as it rounds the corner.
The Underhand (Right-Click): If you are sneaking up on an angle, look slightly up and right-click toss the flash. It travels a very short distance and pops instantly in front of you. Turn 180 degrees to avoid blinding yourself, then immediately swing with the pop.