In most Advance Wars games, there exist a CO Power Meter, which fills during combat:
In Advance Wars, Black Holy Rising and Dual Strike, it increases when any of your Units deal or suffers damage due to combat(direct or indirect alike). However, damage from CO Powers are not counted(Drake's, Hawke's, Von Bolt's etc, which deals damage to enemies out of combat). Altho, Eagle's COP and SCOP can be devastating if used recklessly: as a CO's Power Meter fills faster if suffering fatal casualties, especially expensive Units like Bombers, Carriers and Megatanks.
In Days of Ruin, the CO Meter has returned, however slightly changed:
A single Unit can "board" a CO, then move around with it, boosting itself and neabry Units in an CO Zone, different CO's having different sizes and effects of their Zones. Any of the CO's Units dealing or suffering damage while inside the Zone charges the Power Meter: when half is full, the Zone widens by 1 space, similiar to how Grit's Indirect combat Units gain +1 range when using his CO Power. when CO Meter is full, the Zone widens by another space, and when selecting the Unit which has boarded the CO, a "CO Power" option is available(Forschyte and Caulder is excluded, due to them not having a CO power, not even having a CO Power Meter at all). A drawback is that if the Unit who Borded the CO is destroyed, the CO Power Meter resets, so it is good to prioritize the destruction of an enemys CO Unit, while making sure to protect your own.