Let us say that there are 5 eatable items placed at the dining table during lunch that A is having. From the perspective of A, he has 5 choices and he can choose to eat whatever he wants to eat out of these items. In this way, A has limited free will.
However, the choice that A makes to eat something is governed by the thoughts coming out of A’s mind. These thoughts come out due to the laws of nature of which A has no control. Thoughts automatically come out of the mind, processed by the intellect and the decision is made. All these things happen automatically. In this way, free will is not really free as laws of nature govern both the origin & processing of thoughts.
Mind-shield (Manomayakosha) is the barrier we form as children to protect ourselves from unpleasant feelings like hurt for example. This is because we believe that everything – including those feelings – is real. Once we realise that the reality is pure consciousness and what is not permanent is not real, this barrier of Manomayakosha must go making way for the bliss to shine. Manomayakosha acts as a barrier preventing bliss to shine which is our essential nature. It is okay to be somewhat vulnerable to feelings once it is known that the reality is non-dual. Even if self-realisation has not taken place, but just this understanding takes place that fears are behind many acts that we unconsciously do and it is those fears that prevent us from experiencing ever present bliss, we are ready to work towards diluting the shield of the mind (Manomayakosha) via self inquiry. Diluting Manomayakosha via self inquiry means finding the root cause of any feeling that arises which is of the nature of hurt, anxiety, approval seeking, low self-esteem etc.
PS: Fear is not the only component of Manomayakosha. Desire is also another major component. The above write-up concentrates on the fear aspect.