The Sanskrit word karma stands for the notion of action. Karmic residue is taken to mean the output or effect, in ancient India taken to mean ‘the remainder’, of action. Ancient Indians suggested, then claimed and then believed fanatically (to this day) that the karmic residue of a life, i.e. of a biological unit, as balance of good and bad actions, carries (itself) forward as start-up ‘setting’ of a new life/bio-unit rather than merely as a ‘setting’ change for an already existing life. Verifiable evidence for their claim has not been produced. That karmic residue includes some part or all of its creator, i.e. the creator’s self (or soul as ‘driver’), i.e. his or her essence or substance, has not been observed.
Basically karmic residue happens/activates as observer (or receiver) effect. That’s because ‘the meaning of a message (i.e. of an output of action) is the response it produces.’ Which means that karmic residue becomes active only if and when it strikes as a random (meaning differential) input. Whether or not a unit or quantum of karmic residue strikes and the (good or bad) effect it produces depends on the state of the receiving unit.
In simpler terms, a biological unit (the sum total of all inputs of karmic residues, acting as conditions, so the Buddha) serves as (a self-driving/perpetuating, self-regulating and self-adapting) residue transformation machine.* It inputs** (karmic) residue (recently called ‘garbage’) in order to stay alive, in the process generating new (karmic) residue*** (i.e. garbage) as output. This view was first proposed in the Brhad-aranyaka Upanishad (approx. 600 BC). According to archaic paddle-your-own-canoe, i.e. Hinayana, Buddhists, such a self-regulating conditions (i.e. karmic residue) transformation machine has no apparent (abiding or permanent) self ≈ driver (i.e. as self, spirit or soul).
Elimination of karmic residue results in moksha.
*… The term machine should here be understood in the sense of a Turing Machine, namely as a set of abstract (transformation) rules (or conditions).
**… i.e. feeds on
***… i.e. fodder