Its purpose is to degrade the enemy's ability to fire and maneuver. The target's Aim suffers a 30 point penalty, and your soldier gets an Overwatch shot on the target if it tries to get away. It uses up two units of ammo and cannot be activated if you have less. If your Heavy has Danger Zone and suppresses multiple targets, he'll get Overwatch shot on any of them, but all his suppression effects end after that.
Taking a reaction shot on Suppression does not deduct additional ammo on top of what was already 'paid'. To think of it another way, two volleys is how much is needed for a long-term suppression, but if they provoke Overwatch, you'll instead use some of the ammo to shoot them and then end the suppression. Therefore you can take a reaction shot on suppression even if suppressing used the last of your ammo.
Suppression allows you to pin down a target that you can't immediately kill, reducing its ability to hurt your soldiers. You can then get your people to cover, or attempt to flank the alien. Suppressive fire sometimes destroys the enemy's cover, which may allow another soldier better line of fire on the alien, and tying it together with Holo-Targeting (a passive Heavy ability that adds +10 Aim to anyone firing on units he shots at that turn) and / or Flush (an Assault attack that forces the target to move whether it connects or not) can allow for quick decimation of your target.
Since it affects the enemy's Aim, Suppression is not so effective when used against an enemy that relies on hand-to-hand (melee) combat such as Chryssalids or Berserkers. However, the free shot still applies should they move, and all other races tend to stay put no matter how slim the chance of the Overwatch shot actually connecting.
Using it on the Sectopods allows you to move your men into place without having to worry about their powerful reaction shot.
Dealing with Suppression
Suppression can be dealt with in a number of ways. Firstly you can attack the suppressing unit before moving or using the suppressed soldier. You don't need to kill them, taking any damage cancels the Suppression effect.
The Assault class ability Lightning Reflexes will counter the first reaction shot in a turn, so if your Assault soldier has this ability and is suppressed, it is safe to move him/her. Activating Ghost Armor will also break suppression on that soldier.
Using an ability without moving will not trigger a reaction shot, and many abilities don't use aim, so are unaffected by suppression, such as thrown explosives, launchers, or psionic powers.
A unit being suppressed will have a large "Suppressed!" notification over the movement square icon while selected, to remind you of the risk.
|XCOM: Enemy Unknown: Gameplay Mechanics|
|Action System • Movement • Chance to Hit • Cover • Critical Hits • Critical Wounds • Damage • Flanking • Overwatch • Suppression|