A little doubt:

- I have a Secret: "Make an Ability check in X against an opponent Passive Ability Y".

This Secret is my turn action in an extended conflict. The opponent can make his turn normally or his check is imposed by my Secret, so he cannot act?

Depends on the situation. Usually when a Secret says something like "Do this thing by making an Ability check X, the opponent may resist with Y." it means just that - the opponent may opt to resist, and if he does, then he resists with Y. So just like if you didn't have the Secret, the opponent has a choice: either he resists what you're doing, or he does something else and suffers your action's consequences in full.

For example, if your Secret was Secret of Telepathic Migraine and it simply allowed you to cause Harm to the opponent by using your Telepathy (I) Ability, then you'd use it exactly like you would use a Sword-fighting (V) Ability: the opponent gets to use their own telepathy in an opposed check, or they get to try to bash your head in with a hammer as a parallel action, or they'll make a defensive action with Resist (R). Their choice (but of course you get to change your action if you don't like the match-up).

A Secret may also say that it causes a "reactive" or "reflexive" resist check, which is probably obvious to you if you've fiddled a lot with typical rpg mechanics: such a check would be in addition to whatever else the opponent might be doing. I can remember one place where I've used that, and that is in the World of Near in describing how Moonmetal equipment reacts to interacting with wood: the user of the Moonmetal equipment gets to make an additional check to force his equipment to keep its form.

Finally, a Secret can outright limit the choices an opponent has in answering to action. One of my favourite Secrets in TSoY is the Qek Secret of Sudden Strike: the opponent is limited to resisting a physical attack with a Passive Ability. When I intend a Secret to do something like this, I explicitly say it.

In the situation you describe it is very likely that you intend the first of the above situations: the Secret gives the character some strange and exotic attack that specifically cannot be resisted by just any old Ability, but rather by something specific. A psionic attack could only be resisted by willpower (Resist Ability, that is), for example, and not by dodging it or hiding behind a wall. Even then remember that the Secret text is likely not normative above the fictional meaning of the events: the Secret is telling you that at least this Ability can be used in resist it, but if an opponent somehow happens to have some other Ability that obviously would fit the situation, then that may be used as well. You can see this type of mechanics in TSoY Threecorner magic, which explicitly says that Threecorner workings can be opposed with Resist (R); this is not meant to be exclusive, so if a character happens to have some exotic countermagic Ability (from some other culture, perhaps), then that should work as well. I could imagine being normative about this sort of thing in a very constrained and minimalistic crunch landscape, but then I'd also be very clear in the Secret description that I really intend for only this one Ability to be available for resisting the attack, and if the character doesn't have it, then he's screwed. (Or then he's immune to the attack, could be - I would probably think myself that robots that do not have willpower are actually immune to psionic attacks, not defenseless.)

Thank you very much! ^^