It's possible you're thinking of a different game or mechanic:
* "Dragon Ball Z: Budokai" series: In this series, you can use Ki Blast techniques that appear like rapid fire. The "Rapid Fire" term is not explicitly used.
* "Dragon Ball Xenoverse" series: In this series, you can unlock special skills called "Super Skills" that have rapid fire effects.
To clarify, please tell me:
* What specific Dragon Ball Z game are you playing?
* What exactly do you mean by "Rapid Fire"?
Once you provide more details, I can give you the most accurate and helpful information on how to unlock this feature.