There is a class of magazine aimed at fanatics for some things. Let's say its steam trains. Or dolls. You gin up a mock-up magazine, and sell advertising space in this proposed magazine, predicated on the target audience. When you've sold enough to be in profit, you go into print. Typically there is a free gift in part 1, or a model to be constructed from parts included in the magazine with an implication parts 3,4,5.. will be published in due course. A common sell was to include part 2 with part 1 free. It somehow built belief.

If a significant proportion of magazines sold resulted in a subscription you might go ahead but in practice you didn't bother printing volume 3 onward.

My contact from the biz said they'd repeated this model many many times.