Secret check
Also: Secret time check, Hidden timing point
A secret check times you at a point you do not know in advance. It forces you to keep to the average all the way.
A secret check is a timing point whose position you do not know before you drive. It usually sits somewhere inside a regularity test and measures whether you are exactly on schedule at that spot.
Why secret checks exist
If only the start and finish were timed, you could dawdle in the middle and catch up at the end. Secret checks stop that. You have to hold the set average speed the whole way, because any point might be measured.
How they measure
Depending on the event, the check uses a light barrier, a pressure hose or a marshal with a clock. More and more organisers use GPS timing, where the timing points only exist in software. Then there is nothing to see by the road at all.
What it means for you
You are not driving towards one finish line. You hold the target average all the time. The co-driver keeps comparing distance covered with time elapsed and calls out whether you are ahead or behind. Every second of time deviation at a secret check counts just like at an announced one.