newbie questions on black water tank and campgrounds

2011 jayco featherlite. I was reading the owners manual for the 2013 model as I have not picked up my camper yet. im real new and dumb so far with all these terms. I understand that the black water holding tank is from the toilet. heres my question, the book says that when hooked up to hookups at campground that I keep the holding tank release closed until its 3/4 full and then release it.. im confused.. how does the black water hookup work? I understand if im at a site without hookups then all the toilet stuff goes into the holding tank and when I leave, or when its full then I drive to the dumping station and hook it up to dump it.. .but im confused on when im using full hookups. when im using hookups does that mean I bypass the holding tank? or does the toilet still flush into the holding tank and then from the holding tank out to the sewer system? and if this is the case why does book say to keep it closed till 3/4 then open it? if that's the case it seems like even during full hookup site I still have to fill up holding tank then release it? I was under the impression that the point of a full hookup site was that so when I flush the toilet it leaves the camper right then and there and doesn't go to holding tank... im confused..LOL