The Air Traffic Control (ATC) simulation is one subtest of the ATSA test. It is a game of collision avoidance and basic math questions. You are shown a screen with numbered dots coming in from all angles and your job is to prevent the dots from colliding. When you identify which dots are going to collide, you can press the number of one of the dots, making it disappear, thus preventing the collision.
The ATC simulation test involves different difficulty levels, while in the end the speed of the dots is very high, and you must answer math questions while trying to avoid the collisions. It is considered one of the most difficult parts of the Air Traffic Skills Assessment.