X-Saber Airbellum: This effect will only activate if you attack your opponent directly and successfully inflict damage. If your opponent took 0 damage because of “Waboku,” this effect will not activate.
Judge Program Forum Rulings
These Ruling(s) have not been confirmed as applicable to the TCG.[Notes 1]
These TCG rulings were issued by Upper Deck Entertainment and have since been deemed unofficial by Konami. They were previously considered official and can still be correct, as long as they follow the official gameplay rules, or unless Konami has issued a ruling that says otherwise. References: 
"Waboku" makes Battle Damage dealt by your opponent's monsters to your monsters and to your Life Points become zero.
"Waboku" applies to ALL of the monsters on that player's side of the field for the entire turn.
To illustrate how "Waboku" works, here are some scenarios and their results:
When a monster declares an attack, the game allows for one Chain to be built that responds to the declaration of the attack. This is the opportunity for cards that activate in response to the attack, like "Mirror Force" and "Sakuretsu Armor". After this Chain has resolved, or if both players decide that they do not want to activate effects in response to the attack, the opportunity to activate cards like "Mirror Force," etc. is lost. Effects that do not specifically activate in response to the attack can still be activated during the Battle Step, like "Waboku".[Q&A]
Jinzo: If your opponent activates "Waboku" during your Standby Phase and you Summon "Jinzo" in your Main Phase, the effect of "Waboku" is still applied because it resolved before "Jinzo" was Summoned.
Last Turn: "Waboku" can be activated before "Last Turn", or be chained to the activation of "Last Turn", and in either case the effect of "Waboku" will prevent the activator's monster from being destroyed. This can lead to a DRAW.
Spirit Reaper: "Spirit Reaper" only forces your opponent to discard when "Spirit Reaper" attacks directly (such as when your opponent has no monsters on the field), AND when it inflicts at least 1 point of damage to the opponent's Life Points. So if your opponent uses..."Waboku"...to prevent the damage, "Spirit Reaper" does not force the opponent to discard.
The Dragon's Bead: You can only activate the effect of this card to negate a Trap Card that targets a face-up Dragon-Type monster, so it cannot negate..."Waboku"...
The Unhappy Girl: If "The Unhappy Girl" is in attack position and her controller uses "Waboku"...to take no damage from the attack, the attacking monster still cannot change battle position.
Theban Nightmare: If you have only "Kuriboh" in your hand, and no Spell or Trap Cards on the field, and your "Theban Nightmare" battles with a card that has higher ATK/DEF, and you discard "Kuriboh", the ATK of "Theban Nightmare" is increased by 1500 AFTER damage calculation (but "Theban Nightmare" is still destroyed by damage calculation unless an effect like "Waboku" had been activated previously during the turn).
↑Konami Judge Program Rulings are considered unverified, as their source(s) are not publicly viewable. They are not confirmed as applicable to the TCG until they have been officially published by Konami.
Out of Date
↑"Waboku" does not have a specific activation timing (it can be activated outside of the opponent's Battle Phase). If activated in response to an attack, that attack is not negated. It only prevents battle damage (not all damage), also it only prevents the player that activated it from taking battle damage and only prevents that player's monsters from being destroyed by battle. It does not flip face-down monsters face-up, but they will be flipped face-up before the Damage Step if attacked, even after "Waboku" is activated. Also, it does not end the Battle Phase.