One aspect of our being-in-the-world is that we share the world with other people. Heidegger, differentiating it from individual experience, describes these others collectively as the “they”. According to Heidegger one way you become inauthentic and feel alienated from yourself is becoming lost in the "they" (or lost in a crowd). What happens when you become lost in a “they” that is substantially different to you (e.g. Parkinson’s sufferers in a crowd of those unaffected by Parkinson’s)? The “they” levels off individuality as you conform to what is expected by the others; what is shared is dissolved in the “they”. If you can’t conform due to something outside of your control (e.g. Parkinson’s), then you are defined by this characteristic; what is different remains undissolved and visible. Another “they” in which Parkinson’s sufferers engage is the unaffected person they were in their past and the same dissolving process takes place.
In our thrownness (which may include Parkinson’s) we are just being authentically ourselves. However, being authentic to your thrownness (or physical being) doesn't mean you have to identify completely with certain aspects of your being (e.g. Parkinson’s). Heidegger says that included in our thrownness is an ability to be "ahead-of-ourselves in our potentiality-for-being". Basically, we can't change our thrownness but we have a choice of how we react to our physical being. Its like thrownness has built a house for you to live in; the house has a limited number of rooms but you are free to go into any room you like.
What does Parkinson’s disease reveal about freedom and free will? Acts of free will require two things, a willing agent and a malleable substrate that is acted upon. It feels with my Parkinson’s that I can will my arm to move but it only responds sluggishly. So, my free will is intact despite my Parkinson’s but the connection to the substrate (i.e. my body) is affected; my mind is free but the link to my body is becoming restricted. Therefore, free will can be restricted in itself or free will is intact but the transfer of that will to the substrate is restricted.
Heidegger says that a characteristic of our being is there is always something left to do, something for free will to act upon or try to influence. Even with Parkinson’s we are still compelled by our being to wander around the house of our thrownness, trying to exert our free will.