Forum:Another special summon question I have! Help is greatly appreciated!

Okay so I have a Naturia Deck (and I must say Naturia Dragonfly is godly when used right) and when I use this deck I ofcourse have Naturia Cosmobeet. He's a cool little dude but heres my question

His card simply states that when my opponent normal summons a monster, I can special summon him from my hand. Does this mean I special summon him during my opponents turn, immediately after he summons a monster? It doesn't seem right to activate this during his main phase 1, or during any of his phases. It seems more useful is I special summon Naturia Cosmobeet during my turn, but I don't know exactly what to do. So is it during his turn, or my turn that Cosmobeet is special summoned?

Any help is appreciated =D!!!