That's a great question and I think it highlights a difference between advaita vedānta and trika. Trika views the world as the *creation* of consciousness in a very real sense. It doesn't mean the world is a hallucination. Rather, the world is what comes from creativity / freedom / play.

It's pretty tough to expect most people to accept the idea that the physical world is created from consciousness. But IF you are able to believe that AND believe that the world is real, then it's actually really easy to accept that other types of creations, including the imagination, are also real. That is, there is no fundamental difference between creating something in the gross world versus creating it in the subtle world. Both are equally able to influence the behavior of humans (think of laws, for example, which have no gross reality but still feel real).

So I think when you say "how do we know this visualization isn't just another product of the mind", the answer is that that's the wrong question. Because *everything* is just a product of the mind! Vikalpas aren't fundamentally invalid in this framework. That's part of why speech is seen as so fundamental -- speech can describing things that could never physically exist, therefore speech is fundamentally more powerful than physical reality.

I should mention that I feel like I still have only a shaky grasp on all of this, so I happily defer to anyone who has studied more than me. I probably got something at least little wrong in what I wrote! 😃