I think of it like this. Perhaps every decision you make is /almost/ entirely determined by your current biological state and a long string of preconditions beyond your control. But… there is a sliver of freewill on top of this. And /with/ that sliver, you can choose to make incremental changes to your personal beliefs which molds your perception. By doing this, you essentially alter your biological state in a small way. This effect is cumulative, so you eventually start making different decisions than you would have, which changes your environment, and thus further changes the preconditions of your future. In this way, I believe freewill itself is cumulative. You are born with almost none, but you can develop it incrementally with effort.