Connected to this are Andy Matuschak’s comments about contextual backlinks bootstrapping new concepts before explicit definitions come into play.
What Joel says here about Contextual Backlinks is that they allow you to "bootstrap" a concept (i.e. start working with it) without explicit definitions coming into play (or as Andy would say, the content is empty).
The basic intuition is described well by the Shipman & Marshall paper: users enter information in a mostly informal fashion, and then formalize only later in the task when appropriate formalisms become clear and also (more) immediately useful.
Incremental formalism
Users enter information in an informal fashion. They only formalize later when the appropriate formalism becomes clear and/or immediately useful.