there are fence crossing points not on or close to the straight line, and the planner is forcing competitors to run through them by making them a control (which will obviously be on the map and CDs)
If there is a mapped control at the crossing point then the phrase "not on the obvious line between controls..." doesn't make much sense - because the line will go to that point.
I thought it was intended to be a neat way of ensuring competitors use one of a choice of crossing points (and so avoid annoying the planner / organiser / landowner), but still enabling route choice by not making it a specific crossing point - something that electronic punching can readily allow.