In a nutshell while you are blocking you initiate a command that pushes the attacker away from you to give yourself space and reduce pressure.
Typically while blocking you would push 2 Punches at the same time. (Marvel vs Capcom 2 & 3, Skull Girls)
In Vampire Savior you press P rapidly.
More in depth details
Spoiler: Marvel vs Capcom 2 (click to see content)
Spoiler: Marvel vs Capcom 3 (click to see content)
Spoiler: Skull Girls (click to see content)
Spoiler: Vampire Savior (click to see content)