As snik suggested, not remember when, but here a bit explained
How about make something like this:
[youtube]http://www.youtube.com/watch?v=bE6eNlB23lk[/youtube]
ok, and here how could work
1.Start
2.You have 30 seconds timelimit (or someting) to pass checkpoint
3.If you pass that checkpoint in time, Your time increases so You can reach next checkpoint in time
4.When You fail and don't pass cp in that time, You got eliminated(moved to /spec)