Based on your description, I am picturing a prusik. Is this what you mean (you would tie it differently, but end up with the same result)?
If so, it has the same weakness as a girth hitch, but the reality is that any knot weakens a line to different degrees. It is just good to know that when you are flying with 100lb test, you may only be getting 70-75lbs of strength out of the line because of the weakness at the knot. More of a trivia tidbit than anything else.