From Aozora Wiki
Drives are special items used by the Eleven Weapons. They are long, thin, cylinder-shaped containers with a glass chamber housing a colored, semi-transparent glowing liquid with a black marking on the front side of the glass chamber. Only eleven drives were produced, each with a unique color and provided power. The liquids in the drives appear to never run out as they are brewed with a self-restoring chemical that steadily refills the Drive until it is almost full if a Weapon manages to use enough power at once to cause it to drain completely.
The drives are inserted vertically into the backs of the Eleven Weapons, powering them and their respective weapons up by providing them powers and secondary effects. Since the slot is always exposed with only minimal protection on the outside, the Weapon hides the slot underneath their coat. Because each Weapon prefers to specialize in one type of Drive, the Drives are rarely seen out of their bodies, nor does one Weapon use more than one type of Drive.