He recognised the importance of introspection as a tool of science. It leads to important observations - exactly the tool that FM Alexander used in his observations. De Biran noted the importance of 'the will', of 'willed action'. In this concept lay the understanding of human psychology. The mind operates on the body by 'willing' action.
Muscular activity occurs as a result of the mind 'willing' an action.
The mechanisms for bringing about the muscular activity are entirely sub-conscious. The mind 'wills' an action, and this leads to a circuit that calls on programs at lower levels, which then return to the cortex, to activate the nerves that will act on the neuromuscular mechanisms which will bring about the activity. One could say that we are prisoners of our sub-conscious with the programs that are laid down there. One could say that - but it need not be true. Inhibiting the output of the programs enables more appropriate programs to emerge.