Not long to go ...
The Caballer fireworks firm is responsible for preparing all the rockets used in the running of the bulls as well as all those used in the txupinazo rocket which opens the fiestas. This supplier guarantees that the rockets will always be heard exploding in the sky no matter what the climate conditions are or no matter how the running of the bulls is unfolding on any particular day. Indeed, this would seem to be the only way to communicate with all the runners at the same moment. The two first rockets are fired from the pens to announce that the bulls are on the streets and the last two are fired from the bullring to announce that the bulls are safely in their pens once again at the end of the running of the bulls.
The first rocket goes off to announce that the running of the bulls is about to begin. It also pricks the attention of the bulls as they drowsily pace the pens before the gates are opened. The runners get their adrenalin moving as the quickly calculate how long it will now take before the bulls appear behind them. The second rocket goes off within seconds of the first to announce that all the bulls have exited the pens in Santo Domingo and are now on the street. If there is a longer gap between the two rockets going off, it can only mean that one or more of the bulls has delayed running out and that the pack of bulls is now separated.
When all the bulls enter the bullring the entrance doors are closed and the third rocket is set off. This signals that now there are no bulls loose in the street. The forth rocket lets everyone know that the bulls are now safely in their pens inside the bullring. This rocket means that now the fencing can be opened along the route and the security measures can now be taken off to allow the partying to go on in the street.
If this third rocket delays in going off it means trouble for it indicates that not all the bulls have reached the bullring together and that one or more is still loose on the streets. The runners must be very attentive even though the pack of bulls has already passed them by. A loose bull could try to make its way back down the course and catch the runners in a spot where the fencing does not offer protection, such as at the corner of Mercaderes or the side street of Javier.