Often, resistance has nothing to do with the project itself. You're annoyed and resentful at something that's happening in your life currently, or you're unconsciously stewing because of something that happened in the past.
Your free writing session will uncover this, and then you can let it go.
Sometimes you're resisting because of you're stumped on the project you should be writing. You feel inadequate.
Your free writing will uncover this too, and you can deal with it. You may need to do more research, or ask the client some questions, or do some pre-writing.
I've been working with a couple of writers who are struggling with resistance.
The process I describe above helped them; if you're struggling, it will help you, too.