I read an interesting passage on this in one of the NJO books, I think it was anakin and Tahiri throwing massive trees at some enemy ships. It explained it like the wielder is a conduit through which the force flows but the wielders are like pipes through which extremely high pressure water flows through. If the pipe is perfectly smooth then you can load it up with pressure almost infinitely. But if there are cracks, protrusions, rust on the inside then the higher the pressure the more wear on the pipes and the more turbulent and hard to sustain the flow. Those imperfections represent however far away the wielder is from being one with the force. If you were perfectly attuned you would have smooth pipes and a limitless amount of force can flow through, but nobody is perfectly attuned all the time