I am by no means an expert, but fasting before most drugs does 2 things: 1. Allows the drug to "come on" significantly faster/a bit stronger (alcohol is an everyday example) 2. And, in the throes of a strong drug experience, evacuating your stomach is pretty common. It can be a bit more pleasant if you only have liquids to come up, and nausea generally subsides faster. I can't speak for the salt/dairy/meat ideas. These stipulations sound like they're trying to appeal to their hippie-ish clientele (who might be fans of "cleanses"). Lastly, lots of psychedelics have some cross tolerances, so if you trip on mushrooms or LSD a week before this experience, you might lessen its intensity; so I assume this is why the "no drugs" rule (besides of any unpleasant interactions which might occur). At the very least you are somewhat more prepared for the decent into the fray.