Eh. Watch that
video and you'll understand the concept.
The different skills are triggered by directional inputs (within 15 frames from each other if there are several ? from level2) then shot button, and you can only do a skill if your combo level matches.
A combo level holds during 3 seconds before going back to 0.
Each skill fills your hyper gauge differently, and adds a different delay before your next combo.
The hyper halves that delay.
Hope you enjoy. I do.