fires once intervention hits 100 where it will stay permanently. Likewise, if the Union forces FI to negative 100 it will stay there for remainder of game and FI is impossible.
Once you manage to get FI to +100 intervention does not happen immediately, it takes a turn or two for you to get the notification that France and England have entered the war.
I think Wraith is refering to the chance that one of the foriegn powers will give the CSA subsidies. From the wiki http://www.ageod.net/aacwwiki/Manual:Foreign_intervention
"A positive value for foreign entry also enables the South to get some regular free assets (subsidies)." But you have to scroll through the notices each turn to find the message saying you got a subsidy so it isn't obvious when it happens.
As to "best" ways to fire FI I can't say, but you should know that "In addition, the foreign entry level can shift 1 point per turn in favor of the side with the higher NM, and one point per turn in favor of the side with the higher VP count (50% probability each)." So keep your NM and VP higher than the other guy, and if you're feeling use the political option and try to buy an increase in FI.