This is a prediction run with a staggered start format.
When you register, you predict your finish time for the 5K course. The clock is set to the highest predicted finish time of all the runners and set to count down. As the clock reaches the prediction time of each runner, that runner starts running.
With this format, the slower runners start first, followed by the middle of the pack runners, and then finally the fast runners. If everybody predicts their time perfectly, they will finish at the same moment.
Of course, that never happens, as some runners are overly optimistic with their prediction, while others are overly conservative. Once the last runner starts, i.e., the one with the fastest predicted time, the clock is covered.
Thus runners cannot see the time on the clock when they are approching the finish line, and therefore, don't know if they should speed up or slow down before the finish line. This creates a very unique racing strategy. If you find yourself alone along the course in the latter portions of the race, it means your prediction was way off. If you find yourself amongst a mass of runners finishing close to one another at the finish, it means you've predicted well.
Runners are not allowed to wear watches or have ipods, walkmans, etc.
Note that the clock will be set so that it reads 50 minutes, and is counting down, at 10:00 AM. Thus if you predict more than 50 minutes, you will start prior to 10:00 AM. If you predict 40 minutes, you'll start at 10:10 AM; if you predict 30 minutes, you'll start at 10:20 AM, etc.