The flashbang in Counter Strike: Global Offensive is one of the grenades on offer within the game to be used for strategic benefit. It is the only nade that you can carry two of. It costs $200 and can blind and deafen anyone (including you and your teammates!) for up to 5 seconds when looking at it. It has three types of throw, left click being an overhand throw (the furthest), right click being an underhand throw (the closest) and right and left click simultaneously being in between. It is one of the main tools used in executing a strategy.
The flashbang has many strategic uses and should not be ignored. It can be used to give you the upperhand in an aim duel, to stop a rush, to scare enemies and overall give more map control to you. In many situations it can be the main difference between living or dying. There are four types of flashbang usages.
The regular flashbang throw can be seen for example on A site Mirage, throwing the flash over the ramp from T side. CTs may see this coming with ease and be able to turn around, not being blinded and being able to turn back again to engage in an aim duel or to have visibility of what is happening. This flash is generally not very effective, however can catch out a player focused on something else. They generally do not require any kind of lineup.
The popflash is a flashbang that pops just as it comes within your visibility. An example of this can be seen on Dust 2 mid as T. Walk down mid and press yourself into the corner of x-box against the wall, the side facing T spawn. Line your crosshair above the corner going into the wall and throw, this should pop right inside of the mid doors. Anybody playing mid from the CT side should be flashed, other than people playing deep in CT spawn. This allows you to kill a CT playing here and take full mid control. There are popflashes on every map that can be used by T and CT side, and using them will give you a huge advantage.
The fake flash is a flash that is thrown behind where you intend to push. Using another example on Dust 2 mid, push out to behind the doors. If you wish to pick B door or window, underhand a flash facing CT spawn, while allowing the flash when popped to flash either player. Run in front of it before it pops. It will pop behind you, flashing those in front of you still facing you while leaving you with perfect visibility. If they turn away from the flash you still receive a free kill, as they have turned away from you to dodge the flash.
Pushing through a smoke can be done with a flashbang. Flashes still blind those on the opposite side of a smoke, even if it pops inside of the smoke. This means those watching the smoke for a push will be flashed, allowing you to push through. Throw multiple flashes, if someone sees a flash coming they will look away, however when they turn back they will be flashed by the second one.