Pre-registration for events can become cause for anxiety, with promoters and racers watching the attendance numbers slowly climb. Typically, only half of the racers are registered with one day to go before an event, with the other half registering on the last day.
Adding call-ups to the first 16 riders in each category is an effective way to jump-start registration for an event. Riders are motivated to sign up early when call-ups are assigned in registration order.
Sites like BikeReg.com make implementing this feature easy, since riders can be sorted in order of registration before bib numbers are assigned. At the race start, an announcement is made inviting the first 16 riders to advance to the front.