Sorry for vagueness. I was assuming all use this watch... The subsequent replies hit the nail on the head.
From the instructions
Q What does the efficiency index measure?
A The efficiency index is based on the recognized Swim Golf method that you may be familiar with. It is the number of strokes plus the time taken to swim 25 metres. Concentrate on reducing this number to increase your efficiency and improve your swimming
Tonight I tried to count my strokes in my head. Boy that is difficult. I tried just counting the left hand when it entered the water, but somehow I was counting both hands entering the water by the end of the lap.
I suppose that this will get easier as I practice more.
Anyone got tips on best way to count in your head strokes?