Activate only when a face-up "Neos" Fusion Monster you control is destroyed. Special Summon 1 "Elemental Hero Neos" from your Deck in Attack Position. It gains 1000 ATK and is destroyed during the End Phase of this turn.
You can't activate Reverse of Neos in the Damage Step. (I wish they had allowed it to be used there, especially given it was usable there in the anime and wouldn't have been even close to overpowered even back then.)
The only time you can use it is if your "Neos" Fusion was destroyed by card effect outside the damage step, and that effect was the last thing to resolve. (Being an optional "when" effect, it won't work if destroyed in the middle of a chain.)
So basically, it'd have to be a card like Special Hurricane or Widespread Ruin.