This used to be a somewhat secret method for pulling in good amounts of traffic on the cheap. It can still be effective today, but there’re more "gotchas" to take into consideration before you buy that expired domain name. I’ll explain why this is so a little further on.
First let’s talk about the method itself…
Webmasters allow their domains to expire for a variety of different reasons – what’s important for you is just how much work that webmaster already did to get his site ranked in the search engines, build up incoming links, etc.
What happens is this:
Someone starts a site and begins getting traffic based on the keywords they’ve ranked on, the incoming link relationships they’ve built, any articles they’ve written with their domain in the byline, etc.
But if the domain is allowed to expire – and no one else snatches it up – there’s going to be a window of time where the site is still receiving traffic from those old links and listings.
The traffic that arrives via outdated links will click away unless someone takes over that domain and puts something there worthy of looking at.