The exercise Clockwise was developed to train your attentiveness and alertness, and is actually derived from an experimental simulation of long term monitoring by radar operators in the Air Force.
The task of this exercise is to observe the dots in the clockwise flashing sequence that the hand is pointing to. Similar to a clock. Whenever the hand skips a dot, you must react by clicking "Jump" or by pressing the space bar on your keyboard. Your full concentration is required when the speed and the amount of dots increase.
The difficulty of 'Clockwise' is less the recognition concerning the skipping of dots, but the longterm strain on your ability to concentrate and be attentive.