Pretty much what r0kk3rz said. The key things here are the hardware monitors and how the device reacts to rogue activity. The Neo900 can watch the radio for any unexpected activity, it can watch the CPU-modem data link and knows if it becomes active when it shouldn't, and it can also watch the power consumption of the modem to detect if it's being used when it shouldn't be.

Additionally, the input audio of the Neo900 modem will be disconnected. This means the input audio has been separated so the modem has to ask the CPU when it wants to use the audio channel. It's able to detect if the modem goes rogue and act appropriately.

EDIT: Don't get me wrong. Despite my criticisms, I think the Librem 5 would be a great product and I want to see it succeed. There's nothing else like it on the market today and if it was available now I'd buy it in an instant. I'd probably be prepared to pay even more than what they asking for. That doesn't mean I'm going to hold back from pointing out the lies and bullsh¡t in their campaign. I'd love to support it but I can't. $600 is too much money for me to risk on a product that doesn't exist. It's a shame they don't offer a pre-order deposit of say $100 to allow more people to support them.