Do i owe anything if my car accident case goes nowhere?

So my understanding is that with a contingency fee arrangement you only pay if you win. But what does "winning" actually mean here? If the lawyer takes my case and the insurance company lowballs me and we reject it and then nothing happens, do I owe for the lawyer's time? What about if we go to trial and lose?

I keep hearing "you dont pay unless we win" in every ad but I want to understand what the actual worst case looks like for me financially.