Rapid Fire Cancel is a simple trick that Lucia gets a lot of mileage. In its simplest form, it is a convenient way to quickly traverse flat sections of terrain, spamming Rapid Fire is one of the quickest forms of movement and is available right away.
Any follow-up action that can be cancelled from Rapid Fire can also benefit from it's cancels, allowing you to get into Air Raid state quickly, or use on-landing attack such as Lush which is one of the best moves that builds DT gauge quickly. You can also use Air Hike to keep up the momentum and re-adjust the direction that you are moving at.
To perform Rapid Fire Cancel : Lock on → Direcion → Jump → Attack → Attack → Pause → Attack → Attack → Pause → Attack → Attack → …
Make sure to hold Lock off to ignore enemies. You also need to ensure that you are holding Lock on and the direction stick while spamming the Melee loops.
Example of direction change with Air Hike