There is a lot of marketing power in having a sold-out event. It's the velvet rope concept - as long as there are a few people who can't get in, it makes the ones who got in feel more special. That means as the years progress, people will make more and more of a concerted effort to get in earlier for fear that it will sell out and they won't be able to participate.
There's a technical term for this. It's called Panic.
The main idea is you want to create just enough panic to get people off their butts and into registration, but not too much panic that it causes mayhem. That's where the concept of showing remaining slots comes in handy.
imATHLETE gives you the ability to show the remaining registration slots available for the entire event and/or for specific classes. It would look something like this to the registrant:
[for total EVENT slots remaining]
[for specific CLASS slots remaining]
That's all there is to it!