Chart, page-932

Think of 2 armies fighting on hilly terrain.
When your army gets defeated, you retreat.
You retreat to higher ground on the next hill (resistance point), on the other side of river (trading gap), etc.
You do that because you know your allies will do the same, and you can mount defense there.
You don't want to fight, while your allies are retreating.
You will most likely become prisoner of war (have large losses) and have 2 options:
Cut your leg off and escape (sell at a big losss) or wait to be liberated by allies (break even) which can take long time.

Now. Once you retreated to to top of the hill, and stop enemy advance.
You mount a counter attack.
If you manage to push back sufficiently, enemy will rattle and flee to their own hill.
You know and they know that cavalry is coming (traders who waited for confirmation, investors that are encouraged, etc)
Your enemy doesn't want to get captured just like you and they will offer minimal resistance between resistance points. So you charge at them full power.

The psychology behind SP movements, resistance lines, retracement lines, gaps, etc. Is exactly the same as trenches, forts, defensive lines in the war.

Hope that helps