my general policy was to explain only as far as necessary to get someone in the door, but football is a little more spread out than tennis anyway, I think. obviously you've got to go over Fedal and explain that about 75% of the fandom is just that pairing (oh, I meant to offer when I saw you were doing tennis, this (http://meretricula.livejournal.com/105785.html) might be helpful), but after that... I think you can indulge your own tastes a little. ;)