The offering is completed by an action that is easy to miss and carries the whole doctrine of the rite. Water is poured slowly from a vessel into a cup until it runs over and spills, while the monks chant verses asking that the merit flow to the departed as rivers fill the sea. The overflow is the image: something is being deliberately allowed to run past its container and go elsewhere.
Merit transfer of this kind sits in an interesting position within Buddhist thought. The strict reading of karma is that actions are the property of the one who acts, and no one can take on or hand over another's account. Merit transfer appears to say the opposite. The tradition's own resolution is that the dead person's share comes from their approval of the gift rather than from a straight assignment of someone else's credit, and that the ability to benefit depends on where they have been reborn, with some destinations able to receive and others not. The commentarial literature works this out at length, which is a sign that the tension was felt rather than overlooked.
The rite is repeated. In Sri Lankan practice there is an almsgiving with the offering of cloth at the seventh day, at three months, and at the anniversary, each time with the same pouring of water and the same dedication. That gives a family a definite thing to do at intervals for years, and it is one of the more effective pieces of pastoral design in the tradition: the obligation is finite each time, and it does not end.