Consider William James' murderous mountaineer. His buddy is hanging on a rope that our antihero is holding, and our antihero decides to murder him by letting go. The thought of what he's about to do makes him so nervous that his hands start shaking and let go of the rope. The intention, and by extension the reasons behind the intention, caused the murderous mountaineer to let go, just as he intended to. But although the reasons and the intention cause the letting go, he didn't intentionally let go and his letting go wasn't done for a reason, though it was because of a reason.
This is a famous example where we need the idea of "causation in the right way". Not every intention that causes an action according with intention causes it in the right way, in the way that makes the action intentional. The problem of having to add a "non-aberrancy" or "in the right way" condition plagues a lot of philosophy. A usual thought about such cases is that there is a messy story, beyond our ability to specify all the details. Perhaps that story includes various messy exceptions for various kinds of accidentality, or perhaps it has fairly onerous conditions on the details of the causal chain.
But what if in some--it's too much to hope that in all--cases instead of a long and messy story, we just have a bit of irreducible (or relatively so?) metaphysics. It's just a metaphysical feature of some instances of causation that they are intrinsically non-aberrant.
How could that be? Think of a causal power for an effect as something that can be actualized partially or completely. When a causal power is actualized completely, that causal power automatically causes its actualization, and everything constitutive of that actualization, in the right way. When it fails to actualize completely, it falls short of causing in the right way, though perhaps we can say something more (here's one place serious work would need to be done) about the degree of aberrancy in its partial causes.
It's a medieval dictum that causes contain their effects. But that needs qualification. Causes in a sense contain their proper effects. They contain those proper effects as telê, and then some aspect of the effect--perhaps with cooperation or thwarting from other causes--just is an actualization of the cause with that telos. When all goes well, the whole of the teleologically specified effect is an actualization of the cause, but in aberrant cases, very little is. For instance, in the case of the murderous mountaineer, thinking about how to drop the buddy is an actualization of the intention, but the dropping of the rope is not. There is no further messy reductive story. The one event just is an actualization of the causal power and the other just is not.
But there is something incredible about this story. Sam leaves money for her grandchildren invested wisely in some investments locked up for twenty years after her death. All goes according to plan: the investments rise in value and eventually enrich her grandchildren. But how could the enriching of her grandchildren twenty years after her death somehow have as an irreducible feature its being the actualization of her intention? (Quick thought: It'd be very hard to get a presentist story about this. But presentism is false.) By the time the enrichment happens, her intention is long past. (Does it matter that it's long past? Probably not, but the story is more vivid then.)
There are, I think, three things I can say about the incredulity objection. First, I could bite the bullet. Her intention in some sense lives on in the effects. Yes, these intended effects in the future really just are actualizations of her intention. That's just a metaphysical feature of them. This isn't all that crazy if one believes in the essentiality of origins. For if one believes in the essentiality of origins, then the enrichment's having this cause is an essential feature of the enrichment. Somehow this makes it less surprising if in fact the enrichment is an actualization (or part of the actualization) of the intention. We could even think that the very being of an effect is its having-been-caused.
Second, we could say that when x causes y in the right way, then being-an-actualization-of-x is an intrinsic feature of y, a feature that is causally involved in everything y does, and so when y causes z in the right way, z has the intrinsic feature of being-an-actualization-of-y, and we can go back down the chain to x. Perhaps this is what Aquinas means by per se ordered causal series.
Third, I could go the road of caution. I could say that this metaphysical "actualized by x" feature only is found in immediate effects. Thus, we would in the first instance only have a story about causation-in-the-right way for immediate effects. And then we would use this feature to help construct messier account of causation in the right way for remote effects.
All of this, though, requires a fairly non-reductive metaphysics of human beings.