By putting a card under a specified rear-guard face down from the appropriate zone, it becomes an equipped gauge in that unit's gauge zone. 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.
Not even a dominating player can look at the information of the equipped gauge associated to a dominated unit.
You can place as much equipped gauge as you want below a unit; there is no upper limit.