Playgrounds over Paths
Today's standard design wisdom focuses on the design of linear paths. Sometimes they may branch, but they're ultimately a sequential set of steps towards a desired outcome.
Within complex systems, these paths often lack the flexibility to adapt to emergent conditions - users acting outside the "designed boundaries".
Instead of designing explicit paths, APD is oriented towards the design of playgrounds - limited spaces where the boundaries are defined by the mechanics of the system. Within these playgrounds, desire paths will naturally form based on different factors: cost, convenience, bounded rationality, etc.
Instead of designing the "center" and scripting how activities within it must unfold, it designs the periphery instead, leaving the center alone. It hopes to create a free space, that can accommodate the emergent behaviors of its users.