Welp decided to try and use it just to figure it out myself lol. From what I can gather, once you decide to use the move modifier, you get to change any move on your pokemon to any move they can learn from other TMs and TRs. Hopefully this information becomes useful to more new players as there...
So I got a move mod on blue server a few days ago. Been asking in chat for day and no one can tell me what exactly it does. Don't want to use it without knowing it's potential