In Stross's Merchant Princes series, the US Government uses bits of a world-walker's brain in vitro to power ARMBAND, which lets their planes cross to Gruinmarkt (World 1).
How does ARMBAND "know" to go to Gruinmarkt? We know by the end of The Revolution Trade that there are many possible knots, corresponding to many possible transition edges on the world-graph. What triggers ARMBAND to select that one (or any one)?