The Vulcan Punch is Ralf's unique ability.
In Metal Slug 6 & Metal Slug 7/XX, Ralf Jones can use this ability called the Vulcan Punch that sets out a series of punches. It is very powerful when used, and can even destroy vehicles in a couple of punches.
It is also worth noting that Ralf also used this move in the King of Fighters tournaments.
This attack appears in Metal Slug Defense and Metal Slug Attack too, as a close range attack.