I found this information on a while on the San Fermin website. Hopefully it answers some questions for you. The website is
www.sanfermin.com. Its a very cool site.
There are some age-old activities which have become an essential part of the Fiesta and which are unchangable and untouchable. But from generation to generation some new fads come and go with the years. In the last few years one such new fad has been taken up at the fountain in the Navarreria. This was begun by some Australian visitors who used to meet up together at a bar -The Mejilloneria - near the fountain.
They got the idea of climbing up the St. Cecilia fountain and diving off the top into the waiting arms of their mates below. There is no fixed hour for this action - it just takes off spontaneously at any time during the day and anyone can do it. But it can be dangerous, if the bodies waiting to catch you below should stumble or slip up, you will hit the hard flagstone with a thump. There has already been more than one serious accident here.
Even if you don´t want to jump, it´s worth having a look at these guys - mostly foreigners - doing their own peculiar thing during the San Fermin Fiesta. Just beside the Fountain, you can get a good view from one of the classic old bars of Pamplona - The Meson. Worth a visit.