Not all schools wait until April 15th, and not all schools rank waitlists at all. I know some schools aim for certain percentages of demographics; for example if someone with large animal interests drops then they will replace them with another person with large animal interest.
UMN probably waits until April 15th to streamline it; a new wave of acceptances goes out so they can prepare for the influx of emails and questions. Everyone gets updates all at once instead of having to update waitlist positions constantly. There might also be a chance that someone could change their mind after declining but before April 15th, but I don't know if UMN actually allows that. I kinda doubt it
In short: you are correct. In more detail: UMN waits to pull people off the list until April 15th. They usually do another wave a couple of weeks later when those offers are due to respond. After that they will have one more wave or switch to pulling people as spots open up. And yes, it goes in rank order. #1 is called first, then to #2, and so on
Can I ask why you are waiting to decline the spot if you know you wont be taking it? It doesnt really matter, I'm just curious