Assuming you mean 'Electronic Counter Measures', and the pods and accessories common to both real life aircraft and virtual simulations, then it's a complex but interesting system.

One of the most memorable things I ever heard about ECM is; "While it will throw off missiles trying to lock onto you, it has the same effect as strapping a giant 'EAT AT JOES' neon sign to your aircraft." This is because an ECM pod sends out the very signatures that a radar is looking for, which may sound silly until you understand what it's doing to those signatures.

The offensive strategy of ECM is 'jamming' a radar, which requires finding the radar receiver - the part that picks up the emissions that the radar is giving off to detect targets - and saturating it with information. Take a ball-and-cup game as an example. The player tosses the ball upwards, where it hits the end of the string or gravity turns it around (which is where the radar wave hits the target and returns.) Now, the player is trying to get the ball back into the cup, which in this case is really easy for him. "Jamming" would be comparable to suddenly tossing dozens of golf balls, tennis balls, and basketballs at the cup when he is trying to catch the one ball he has.

Jamming, in effect, overloads the particular frequency, or frequencies, of the radar. This is an example of 'Spot jamming' pouring all the ECM's power into one frequency, which severely limits that radars ability to detect on that frequency, making all incoming radiation in that range muddled at worst. Another example is 'Sweep jamming' which uses the same concept, but cycles through a large variety of radar signatures at full power. The downside is that only one frequency can be 'jammed' at any one time, but the practical upshot of this system is that, depending on the design of the target, this may completely remove it's ability to detect enemies.

'Barrage jamming' is the same as 'Sweep jamming', except that it can cover many different frequencies at a single time; however, it accomplishes this by reducing the power in each frequency, severely limiting it's ECM effectiveness in trade for a broader spectrum. 'Base jamming' is a recently developed type of Barrage jamming, which focuses on one radar, leaving others in the area working but completely shutting down that one installation.

Finally, there is Digital Radio Frequency Memory, or Repeater jamming. This is accomplished by changing the signals that are returning to a radar; radars, as you recall, send out energy and read what bounces back to come up with a general idea; size, speed and direction. This type of ECM changes the energy being sent back, through Doppler shift or simple timing. Thus, the target can appear to be farther or closer, higher or lower, faster or slower, and most importantly, NOT where it actually is.

So, what does this mean in the world of virtual reality and the aircraft, vehicles, even HERCULANS and Battlemechs?

HERC A has a radar lock on HERC B. Target B activates his ECM pod on a Spot Jamming system, which affects the area around him, and until HERC A changes his radar frequency - possibly limiting it's effectiveness - he no longer has a solid lock on HERC B. However, HERC C, sitting on the sidelines, can see HERC B as bright as daylight, because he's using a different radar frequency, or even passive radar (Another subject). He gets target lock on HERC B and launches an Active-Radar-Homing (ARH) missile at him. Bad move; the onboard guidance radar for the missile uses the same frequency as HERC A's sensors, and ends up getting jammed, spiraling around at the mixed radar signatures before slamming into the ground.

'Mechwarrior A is being detected by three different radar installations, which he knows all operate on the same frequency at the same time, being linked together. Activing his ECM pod on a Sweep system, changing it's jamming signal to match the radar's, they quickly lose track of him. However, 'Mechwarrior B the next ridge over, who didn't know 'Mechwarrior A was there in the first place, suddenly has a massive radar signature on his hud, and will most likely go to check it out, adding to the first warriors problems.

An F-16 Falcon driver suddenly realizes he's too high in enemy airspace, and three separate radar sites have him locked; they are all using different frequencies. He activates his ECM pod under the Barrage system, and cycles it to all three targets; their missiles still launch and come close, but not as close as they could have had he not reduced their radar installations' effectiveness.

An F-15 Eagle driver gets a tone lock on an aggressive MiG-25, and launches a Semi-Active Radar Homing AIM-7 Sparrow missile (SARH, or by NATO designation, 'Fox one', in case you ever wondered what those wingjocks on the big screen were shooting at foxes for.) The SARH system requires that the F-15 keeps a solid lock on the MiG-25, since the missile doesn't have it's own advanced targeting system, relying on information from it's parent aircraft. The MiG-25 activates it's ECM under the Base Jamming system, targeted on the F-15's radar system to try and shake the missile. Since his sensors no longer have a lock, the SARH missile rockets in the wrong direction, most likely crashing down to the surface when it's propellant ran out without exploding.

The same F-15, on it's return to base, encounters an advanced SAM site, and actives it's own ECM pod under the Repeater Jamming. The SAM site's missiles end up flying almost a mile behind him before detonating in the wrong direction, and the Eagle driver hits the deck before it launches again.

As you can see, jamming is both defensive and offensive. In the case of the more common video games use, which is an 'invisibility to radars and throwing missiles off', it's more of a 'way too many targets to hit the right one' than a true invisibility. Even with ECM on full power, it is possible to get hit; but it sure does reduce the chances!

'Defensive' jamming is a much more precise activity, which uses the same concept to 'hide' or 'mask' a preferred target. In this example, a naval fleet uses Blip Enhancement, making the ECM-equipped ships appear larger. While the enemy wishes to strike at a naval fleet's Aircraft carriers, which would appear to be the largest blips in the fleet, they are confused as suddenly it's destroyer escorts are, according to their radars, now Aircraft carriers. Then it's picket-line of ships are the same size, until the entire battle fleet appears to be composed of nothing smaller than the Aircraft carrier: They cannot launch an attack against what could be the wrong target, and lose valuable time and resources. Or if they do launch the attack, using standoff weapons like long range missiles, it is likely they will hit the wrong target, a patrol boat pretending to be something much more important. Even then, hitting it would be questionable, as it could be that they hit the radar-sized ship, and fire all around a tiny boat.

Hopefully, this is a precise answer to your question. Sorry for the more obscure 'HERCULAN' references, but it suited my purposes. I could be incorrect about some of this information, so it would be best that you check all facts before you go spouting them off again.

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