Kullūpama: The simile of the raft
A raft is worth building and worth boarding, and it is meant to be left on the bank once you are across. Shadow of the good to come describes exactly this sort of usefulness: the law carries people toward something it cannot itself hand them. Trouble starts when the raft gets hoisted onto the shoulders and carried inland, when the shadow is taken for the thing casting it.
Yañña: Sacrifice
Sacrifice, in the Buddha's handling, moves off the altar and into conduct. Asked which offering bears the most fruit, he ranked feeding people, keeping precepts, and training the mind above any beast bound to a post, and the yearly bulls and goats of the atonement fall under the same weighing. Blood that changes nothing in the one who spills it leaves the giver standing where they were.
Vaṭṭa: The round
Year by year, continually: this is the round, the circle that returns you to your starting point. The high priest goes in with blood and comes out, and the same day comes around again with the same need, which is the shape of a wheel rather than a road. Anything that has to be repeated forever has finished nothing.
Sīlabbata-parāmāsa: Clinging to rites and rules
Grasping a rite as if the performance itself purified is among the first fetters dropped at stream entry. Offering continually in the confidence that this year's sacrifice will do what last year's did not is that grip tightening with each repetition. Handled as training, the rite keeps its place; the weight of expectation hung on it is what sends those who draw near home unchanged.
Asekha: One beyond training
Made perfect would mean nothing further remains to be done, the training finished and set down. Sacrifices that can never make perfect keep everyone a permanent beginner, coming forward each year with the same gap unclosed. Completion of this kind happens once, in the person, and needs no renewing.