Crab Apple is known as the cleansing remedy, and as such is the added 'sixth ingredient' in Rescue Cream. Its main use is to help people who feel that they have something unclean or poisonous about them, or who dislike some aspect of their appearance or personality.
Sometimes there might be very real things wrong with someone in this state, but they will disregard the big problems and concentrate obsessively on the one thing that they have fixed on. This is why Crab Apple is sometimes given to people 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.