Old Versions of pokemon

Well, I was looking through my old cards and I found a pokemon power,
that I didn't quite understand...it is for Dark Golbat and it says
"When u play Dark Golbat from ur hand, you may choose one of ur
opponent's pokemon, If u do, Dark Golbat does 10 damage to that
pokemon. Apply Weakness and resistance"
Here is the thing "When u play Dark Golba from ur hand" I understood
that in a sm other different way than my friends did...
I practically though it was meaningless...or, that i cant use the
pokemon power while the card is still in my hand, which is a well-
known rule...or at least, i understood it smhow enough to disagree
with my friends, on the other hand, my friends think that i m only
allowed to use this pokemon power ONCE, which is when i play the card
from my hand..."when ur next turn comes and u try to apply the
conditions (when u play Dark Golbat from ur hand) u will realize that
its impossible to full fill them" says my friend..."Its already full
filled, that card is played...and how come i can use a pokemon power
ONLY ONCE in the whole game" ! i replied...
so, is it only once during the entire game?