It seems lots of people have this ability. I have also been able to voluntarily stop hiccups since my youth, but I use a different approach. I concentrate on relaxing the back of my throat and upper esophagus region. I try to visualize this region and will it to relax. I perform slow steady normal breathing while doing this. By concentrating for just 15 to 20 seconds I can always stop my hiccups. This probably helps quiet any stimulation of the phrenic nerve as it passes through this region.