When you pass down a line that you can see from the main grid, the next line will take you either out of the back area again or to some significant spatial event, such as a larger piece of space or a significant building, within the back area. This means that whereever you go, there is usually a point from which you can see where you have come from and where you next point of aim might be.
Two-line logic
Sources
Hillier, B. (1996, 2007), Space is the Machine: A Configurational Theory of Architecture. Space Syntax: London, UK. p116-118