Five Brothers Explosion

The effect "When this card is activated: Gain 500 LP for each Continuous Spell/Trap you control" is the effect that resolves when "Fire Formation - Tenken" is activated. (This effect does not target. It cannot be activated during the Damage Step.)

When resolving the effect "Gain 500 LP for each Continuous Spell/Trap you control," you gain 500 LP for each face-up Continuous Spell/Trap card you control. (This includes the "Fire Formation - Tenken" that was activated.)

The effect "If this face-up card on the field is sent to the GY by an opponent's effect: Inflict 500 damage to your opponent for each Continuous Spell/Trap in your GY" is an effect that activates in the GY and creates a Chain Link. (This effect does not target. It can be activated during the Damage Step.)

When resolving the effect "Inflict 500 damage to your opponent for each Continuous Spell/Trap in your GY," you inflict 500 damage to your opponent for each Continuous Spell/Trap card in your GY. (This includes the "Fire Formation - Tenken" itself that is in your GY.)