The prizes will be awarded to the first user who registers for the promotion during the winning moment.
This means that if you have defined a winning time and no user registers at this exact time, then the prize is awarded to the first user who registers. That is, the application will try not to leave any prize deserted. An instant win will be unassigned if any user registers between the winning time and the end of the promotion.
If you prefer, you have the possibility to enable void prizes, that is, to leave a prize unassigned if no user enters the promotion at that time. To do this, you must select the 'Enable void prizes' in the Tools > Instant Win section of the promotion Editor: