Forum:Rainbow neos effect

hey im new to this game can i ask effect that says "once per turn"can we use it in opponent's turn? for example rainbow neos,when i play with my friends i used change of heart.and he said he uses his rainbow neos effect to send his rainbow neos to his graveyard to return all monster on my field.can he realy do that?please help me he won't believe me if i told him.