Spread the password and access details to two people without keeping it yourself. Those two people split their in half without keeping it themselves. The four people who now have the password are the 'kill switch' required to monitor Snowden's location. Snowden has no idea who they are.

Should he be tortured, all he can do is reveal his first two contacts. The torturer will now have to abduct those two people to get the key.

All these second level people know is the two people THEY spoke to. So you need BOTH of the second level to get all four of the third level people.

And then you'll have to abduct another FOUR people to get the key. All before these four people realize Snoden has disappeared.

He knows he can't resist torture. But he doesn't have to. He LITERALLY cannot be coerced.