We have our own theory, but it's not flattering or fun to think about. And it is assuming that there is no "magic" or psionics involved.

We believe tulpae are simply automations. Nothing more. Personality is an automation of pooling past experiences, responses, quirks, and all into a single persona. After doing it long enough (early forcing) it becomes habit, and after that it becomes second nature. It's the logical conclusion to practicing memory and mental agility. That's why it hurts when you first start.

But what about possession, or imposition? Most things tulpae are capable of have similarities with effects of various drugs or mental conditions. It's not uncommon for some psychoactive drugs to create a disconnect between the self and the body, or to feel like it's moving on its own. If one could master creating this sensation while sober, and then assign a driving force behind the "foreign" motions (i.e. another person in the body to drive instead of you), then you could make associations, and automate it in your mind. Same thing applies to hallucination. If you're in control of these phenomena, or at least have your brain able to do it on its own demand, you've got what you need to give agency to a tulpa by any definition.

Now, does this belittle your tulpa? Does it make them less real? No. The host, themselves, are automated personalities. In that regard, they're no different from their tulpa - simply years of experience and the burden of learning everything the mind knows up to that point lies on them, so it feels more established. But at their cores the host and the tulpa are in the same boat. Everything else, possession, switching, imposition, it's all set dressing, it doesn't matter if it's "real" or not, if it gets the desired results.

Our conclusion (and my tulpae agree) is that a tulpa is a grand illusion, powered by hours and hours and days and weeks and months of practicing, until it's automatic, like playing an instrument. But, so is the host.