You must have read a Korean version of Slam Dunk. 거들다 positively means to help others, and negatively to intervene in other people's business. In that context, it's positive. Imagine a basketball player holding the basketball. He has his left hand tight on the side of the ball to support it and shoots it with his right hand. So, I would translate "왼손은 거들뿐" as "the left hand is only for a support." And you can understand "~뿐" as "only" or "just." I hope this helps !