Cynosural System Jammer
All this data is potentially out of date, and should be taken with a truckload of salt
Cynosural system jammers are POS modules which prevent cynosural field generators from operating. The exception is covert cynosural fields which can be deployed in jammed systems (though highsec is still off-limits to all cynosural field generators). The term for a system that has an online jammer module is a 'cynojammed' system.
To anchor or online a cynosural system jammer requires the owning alliance to have a Strategic Index level of at least 3 in the solar system, as well as a Cynosural Suppression strategic upgrade module installed in an Infrastructure Hub within the system.
Limits on other POS modules in system
Only one jammer can be anchored at any one time per system. The result of this is that an incapacitated module must either be repaired to operational status or must be destroyed before another jammer can be deployed and onlined. In system sieges, this mechanic can be used to prevent defending parties from simply bringing in another jammer to defend a system after the attackers incapacitate the first jammer.
As the function would suggest, Cynosural Generator Arrays do not function in systems which are cynojammed. Jump Bridge modules still function, and after a system is jammed no capitals from either parties may jump into the system, however defenders are fully able to still use any capitals logged off in space or station in system when it is jammed.