Someone asked: does the wholesome's mechanism for cutting off a karmic result mean using merit to actively cut the sphere of demerit down, or does it really mean growing the sphere of merit bigger than the sphere of demerit so the demerit simply can't produce its result?

Within us, the sphere of merit and the sphere of demerit are stacked, sharing exactly the same center. Whichever sphere is larger envelops the other. The larger sphere releases a current outward that draws in components to itself; as it releases that current and draws things in, it's gradually spent, and the larger sphere gradually shrinks. If the sphere of demerit is larger, it's the one that opens its current first, while the smaller sphere of merit stays enveloped, unable to act. This is why we need to keep making merit, to grow the sphere of merit bigger — as the sphere of demerit gets used, it shrinks bit by bit. The sphere of merit we build up through keeping the eight precepts, practicing Dhamma, and cultivating meditation keeps growing larger and larger.

This mechanism only works when we actually act — it isn't something that cuts the karmic result away in one stroke. As we continually create the causes of merit — giving, keeping precepts, cultivating meditation — the sphere of merit keeps growing until it reaches a point where the sphere of demerit can no longer envelop it. The two spheres then become nearly equal, and once they do, the sphere of merit begins to release its own current outward, turning what was severe into something lighter, what was bad into something better — shrinking the sphere of demerit, because releasing that current to draw things in, or to compel us toward or away from certain places and circumstances, requires energy to operate. The sphere of demerit gradually shrinks as it's expended this way — much like a phone battery that keeps draining the more it's used: if we never recharge it, it eventually runs out, but if we keep recharging it, it never quite depletes.

While the sphere of merit is growing but not yet used, it remains enveloped, unable to act, yet still growing all the while. As we continue creating good causes, the fresh merit draws in old merit that hasn't yet been placed on the timeline of our life — because the unwholesome and the neutral natures interfere and hold some of it back, leaving portions of both merit and demerit not yet placed on our life's timeline. If we create the causes of demerit, we likewise draw in old, unspent demerit — for example, demerit from gambling in a past life. Even if part of it was already worked off, turning from severe to light, some residue can remain; when we then associate with friends or circumstances involving gambling in this life, that old demerit we once created gets drawn back in to produce its result once more.