Faulty sensory application is Alexander's term for using a constellation of bodily feelings as feedback for the quality of movement. Unless we have learned to coordinate our movement using Primary control, our sensory feedback is rather informed by our habits than by easy coordination. What feels 'right', is rather what we usually do than what is healthy or efficient.
There is nothing 'wrong' or 'bad' about the 'feel' of a movement, it's just a very inefficient gauge for the quality of movement. Application of the Alexander Technique enhances the reliability of sensory feedback, yet rather as a side effect of directing than as goal in itself.