Edison Format · March 2010
The damage inflicting effect of "Yubel" is a Trigger Effect that activates before damage calculation.
Tributing 1 monster or destroying "Yubel" is a Trigger Effect that activates during your End Phase. You make your choice when the effect resolves.
When "Yubel" is destroyed while in the hand or Deck, its effect activates. If it is destroyed while face-down, its effect activates.
The owner of "Yubel" activates the effect. (If your opponent controls your "Yubel" when it is destroyed, you get the effect, not your opponent.)
When "Yubel" is destroyed during the Damage Step, its effect can be activated.