first, you can get the boost only by setting to manual.
second , you charge the boost by time or angle (if you do a sharp turn it will charge quickly).
the bikes only have a blue boost IMO (tried it for long turns , a few times).
I think that you saw the ghosts leave the slide button ,and it looks like its just keeps charging.. i could be wrong about it.
My explanation of it , is that bikes have a speed boost while going straight (wheelie) , so they can charge a bit less than karts on turns...