Whenever a Special Forces or Elite Special Forces scout unit moves close to an enemy AA system, it disables it's weapon systems. This works with both stationary and mobile anti-aircraft systems. Affected AA systems show a red Jammed in their infobox when selected, but only stationary AA systems will also have yellow sparkles around them. A Scout currently jamming sends out bluish-white pulses and a distinctive sound is heard.
Note that only the target's AA weapon systems are disabled - stationary defenses will still be able to spot hidden units such as the Scout unit disabling it, while mobile anti-aircraft can simply move out of jamming range.
The only ways to make affected anti-aircraft to work again is to eliminate the responsible scout(s) or wait for those units to move.