Karma is causality when there is doer-ship and this mixes the gunaha, the aspects, of the elements, namely AkAshaha, vAyu, Agni, Jalam, Pruthvi and gives them different natural characteristics. Interactions between the elements must be driven by some force. This force must have been accumulated like the concept of feedback in cybernetics. What is accumulated is karma.
The body is made of up these elements as well. Your thoughts and feelings, too, are made up of these, they just have a different classification, in English we call them subtle. Between the subtle and the gross there is a relationship, and supervenient relationship. Your organs of action need sensory input because they are just output, metaphorically speaking, because for karma to be real, there must always be doer-ship, and these sensory stimuli must cause an action or actions. Well, when the organs of action die, what is left? The sense organs of action. The organs of action merge or decompose with the earth because their action, i.E karma has been exhausted, but sensory organs live on until they exhaust their own karma. (They with out a doubt will control different organs of action or gross elements that are arranged in a way where they are supervenient with the subtle elements.
Because the soul is improperly understood under reincarnation. The idea of the soul as a separate substance, properly without the body, is misleading and erroneous. The human soul is properly united to the body because it understands through the senses of the body and exercises the body's vital functions. It is clear, then, that the human soul is naturally united with an individual body, and is not a ghostly substance which can be transposed to animate another corpse. Rather, the soul has one body; reincarnation is therefore metaphysically impossible.