By putting a card under a specified rear-guard face down from the appropriate zone, it becomes a gauge equipped to that unit. If you are putting the top card of your deck as an equipped gauge, you cannot look at the card before putting it.
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.