BUAs and Barkers

We used a BUA in a campaign game recently (I'm a charter member of the BUA Delenda Est club...so no comments, please!). How does barkering work? Does one opposing unit on a side prevent the troops inside from exiting? If it's a long side, does a barkering enemy unit affect the entire lenth of the side? Does a barkering opponent stand prevent the defender exiting on any or all of the four sides?? Enquiring minds want to know! The occupying troops can teleport through the BUA to exit on any side, so what's the scoop?