I actually worked for 5 months as a guide at QuetzalTrekkers in León, and know the Cerro Negro volcano very well.
The boards should have a smooth section at the back, and the secret to a fast ride down the volcano is to place your weight completely on the back part of the board. The secret to a successful ride down the volcano is to avoid spinning by placing some support at either side of the board near the front (i.e. your feet on the ground).
If you stand on the board, neither of these things happen, and you will slide slowly before falling off Although admittedly, it would be a cool photo!
Some people try to do a zigzag snowboard style on a different board, but it was always frustrating to watch them, because regardless of their skill, the material of the volcano is much too hard (compared to snow or to sand dunes) so unfortunately the Physics just doesn’t cooperate
Other advice would be to go on a tour that lets you go twice if you can - often people are scared the first time but know exactly what to expect the second time!
Also, the pink and blue boards that you can hire at Cerro Negro itself are really poor quality, and it was frustrating watching tourists use those. Ensure you use one of the thicker boards that gets repaired every day. Most companies in León now use these.