The server can force cvars on clients and the server itself to a particular value. This includes cheat protected cvars which otherwise could not be changed from the console. This different from Cvar Restrictions.
Forces all clients to set the cvarname cvar to value. This will not prevent clients from changing the cvar after forcecvar is executed.
Client Cvars you might want to force
remove foliage, gives a big FPS boost on maps like radar.
you really need it set to 0 on the server before clients load, otherwise they wont get the full benefit. you can turn it to 0 mid-game but the client rendering engines have already loaded the base foliage models so you get some reduction but not as much as if if was set to 0 before they connected. if its 0 before they connect, the engine doesnt even load the models at all.
turn off fog. This has a big performance impact, even on maps which don't have much visible fog.
until recently, PunkBuster had hard coded checks for this cvar, and would kick players if this cvar was 0, regardless of any PunkBuster settings. As of November 2005, they seem to have reversed this policy
This may cause very ugly visual artifacts on some maps, or cause drastic changes to visibility.
prevent the display of the complaint popup. This doesn't affect users ability to file complaints, it just controls the popup. This has been reported to reduce 'newb complaints'
Server Cvars you might want to force
allow developer commands / cheat protected cvars without changing the server commandline
show bullet debugging information.
etpro configs can lock server cvars with setl Pluggable Configs and client cvars with sv_cvar Cvar Restrictions
forcecvar may be used to change cvars that the game didn't expect to be changed. This can cause unexpected behavour.