Suppose you want to require a link back in order for anyone to get listed in your directory. To do that, go to Admin Panel -> Settings -> Switches and enable the "Only allow reciprocal and sponsored" switch. If you don't have any sponsorship levels set up, this will act as reciprocal-only. If you're looking to ensure that people have properly reciprocated before they submit (to save you time reviewing) you should also turn on the "Verify reciprocal link on submit" switch.