With wall hack, the Sniper can shoot through solid objects on the map (but not through enemies/corpses). Use it to kill enemies far away from you who try to hide. Has most effect on Street map, as it is split by a wall in the middle. Wall Hack can be used best in Gun Games. That way, Snipers can use all types of weapons and activating the wall hack, explosive projectiles and rockets can travel through objects too. This works on most weapons, but Grenades may not pass through all surfaces.
Justin and Mike use this killstreak against the player and that is considered the hardest part of the mission.