It's a McGuffin
Just as there is no listed rule for exactly what the negative energy plane does to living creatures, the "some rare form of magic" means you as the DM will make up something that fits with your campaign that allows the party to visit the Negative Energy Plane without instantly dying (or, alternatively, you can choose not to do so, making the plane functionally inaccessible).
As an example, in the Exploring Eberron book*, the plane of Mabar (which is a sort of setting-specific mashup of the Shadowfell and Negative Energy Plane) deals 10d6 necrotic damage every minute to any living creature, and if you hit zero HP, you crumble into dust. The rules for this specifically call out that death ward and resistance or immunity to necrotic damage will nullify the effect.
That's not the Negative Energy Plane as such, but it's an example of what you could decide to implement in your game. The defenses could be as simple as those -- making the plane accessible (at least for a short time) using just a few specific spells or items -- or it could be an ancient spell found on a stone slab deep in a mummy's tomb, a magic item that creates a bubble of life energy, or magitech space suits. You decide what makes sense for your game.
That text is basically the game designers lighting up a big neon sign that says PLOT HOOK GOES HERE.
*Exploring Eberron is admittedly a DM's Guild book, but it was written by Keith Baker, the original creator of the Eberron setting. While it isn't "canon" (whatever that means in D&D), it's probably the best insight we have into what the setting is intended to be like in the areas not covered extensively by the main book.