If I am defending alone at Forbidding Waste and I add a unit during that showdown, what happens to the minus 2?

To determine what happens to the -2 Might penalty from Forbidding Waste, we must look at the battlefield's ability and the rules for when a unit is considered "alone."

The Ruling

When you add a second unit to the battlefield, the -2 Might penalty immediately ceases to apply to your defending unit.

Reasoning

1. Ability Condition: The ability on Forbidding Waste states: "While a unit here is defending alone, it has -2 [S]. (It's alone if there are no other friendly units here.)"
2. Definition of "Alone": According to Rule 741.1, a unit is alone when there are no other friendly units at the same location.
3. Dynamic Application: Because this is a continuous effect tied to the battlefield state, it is re-evaluated constantly as the game state changes. The moment a second friendly unit enters the battlefield, your first unit is no longer "defending alone."
4. Result: Since the condition ("defending alone") is no longer met, the -2 [S] penalty is removed from that unit.

Important Note on Timing

If you add a unit to the battlefield during a showdown, it happens while the state is Closed (if a chain is active) or Open (if no chain is active). Regardless of when it enters, as soon as that unit is officially present at the battlefield, the Forbidding Waste condition is no longer satisfied, and the penalty vanishes instantly.

If this happens during combat, ensure you are tracking the unit's current Might correctly for the Combat Damage Step (Rule 460), as that step uses the might values present at that time.