"Armed" likely comes from the aesthetic of the Tachikaze clan. Many units within the clan are militaristic in design, with many units wielding large cannons, guns, and other weaponry. Placing an equip gauge under a rear-guard is likely a reference to "arming" that unit with more weaponry and ammunition.
How it Works
By putting a card under a specified rear-guard face down from the top of your deck, it becomes an armed gauge equipped to that unit. You cannot look at the card from the top of your deck before putting it under a rear-guard.
The equipped gauge under the unit is considered associated with said unit, and can be used for costs or effects.
When a unit leaves the field (such as being retired), or it is locked, all of the equipped gauge associated to it will be put to the drop zone.
Only the original owner of the card can check the contents of the equipped gauge.
Even if a fighter dominates a unit with equipped gauge, that fighter cannot look at the equipped gauge associated to the dominated unit.
You can place as many equipped gauge as you want below a unit; there is no upper limit.