It has a similar function to the Scramblerperk in Call of Duty: Modern Warfare 2. It also has the function of disabling voice announcements to any enemy players within its proximity, and disabling any enemy equipment within its proximity. However, it must be placed at a certain location and will only affect players that are close to it.
Placing Jammers in general is similar to placing claymores in that they should be in a useful location (a choke point for instance), but shielded from prying eyes by some obstacles and the like. Also be aware that the Jammer emits a static-like sound when in close proximity, which will alert enemies to its location unless it is well-hidden e.g. behind debris or other objects. Finally, like any equipment, it can be retrieved (with the 'use' key) after placing it. Generally, Jammers are slightly more useful in Core game modes where everyone has a mini-map at all times, as opposed to hardcore modes, when players only have a mini-map when an allied Spy Plane or Blackbird is active.
It also effects RC-XD's as they will act like they are stunned, one could drop it when they see a RC-XD coming and have time to shoot it.
The equipment scrambling distance is quite substantial, taking up an area approximately the size of the central building in the multiplayer map, Array. In other words, a circle with a radius of approximately 30 feet.
While not particularly effective in Team Deathmatch, it can be used on a case-by-case basis to sabotage a camper's claymore and sneak up behind them without alerting them by destroying the claymore. That said, the claymore does make a slight sound and emit sparks when disabled, so an alert camper may still notice. Also, it makes a great distraction for luring enemies into an area, hoping to destroy the Jammer.
In Capture the Flag, Jammers can be used by offense to neutralize enemy claymores, C4, and block out the defenders' mini-map prior to attempting to grab the flag. Conversely, it can be used by the defenders to scramble the mini-maps of incoming flag runners and prevent them from tracking defenders in that fashion and also prevent flag runners from using claymores or C4 to cover their escape. These tactics also apply to Search and Destroy, Sabotage, and Demolition game modes.
In Headquarters, the Jammer can be used to confuse enemies moving towards the HQ and hide the activities of defenders as well as prevent enemies from assaulting the HQ with C4.
In Domination, the Jammer can be utilized by Defenders to hide them and their allies so the Attackers must charge in blindly, making them easy targets. Equally Attackers can utilize them to disable the Defenders equipment and have them come out of hiding to search for the incoming Attackers, making them easy targets for a well directed airstrike or Valkyrie rocket.
The Jammer, combined with decoys, can be used effectively as a camping tactic. Enemies will near the area of the decoy only to have their radar jammed, and possibly killed by the owner of the Jammer.
Jammer is exceptionally good at getting the kills needed for hacker pro, due to its much bigger range than motion sensor. However, the Motion Sensor has the benefit of letting the player know someone is in its proximity.
Jammer is very useful when used in Array inside the circular building. It is most hidden when placed on the lower floor level.
It is very useful to combine this with Hacker Pro, as Jammer will jam enemy claymores or C4, making it possible to hack them from the front without risk of it detonating. Note that this is useless for motion sensors as Hacker Pro makes you invisible to them.