This is known as the cleansing remedy, and as such is the added 'sixth ingredient' in the cream version of Dr Bach's original crisis formula. Its main use is to help people who dislike some aspect of their appearance or personality, or feel that they have something unclean or poisonous about them.
Sometimes in a Crab Apple state we disregard genuine problems and concentrate obsessively on one thing that we have fixed on - sometimes a quite minor aspect of the whole. We might take Crab Apple to cleanse obsessive, repetitive behaviour such as hand-washing, re-checking that appliances are unplugged, and so on.
Dr Bach's description:
This is the remedy of cleansing. For those who feel as if they had something not quite clean about themselves. Often it is something of apparently little importance: in others there may be more serious disease which is almost disregarded compared to the one thing on which they concentrate. In both types they are anxious to be free from the one particular thing which is greatest in their minds and which seems so essential to them that it should be cured. They become despondent if treatment fails. Being a cleanser, this remedy purifies wounds if the patient has reason to believe that some poison has entered which must be drawn out.