If you are new to this type of event you might not have heard of this particular feature before.
A reentrant is a small valley in a hillside, the centre of which would collect water and funnel it downhill (if it were raining hard). It appears on the map as a U or V shape in the contour lines, pointing back into a hillside rather than sticking out of the hill (as would a spur). Hillsides are covered with reentrants and they are very popular as control features with course planners – especially ours!!