How this rotation runs
At each rotation the guard on break walks to the stand, scans the water on the way, and takes over. The guard who was up goes on break. With a 20-minute rotation each guard watches for 20 minutes and then gets 20 minutes off, which keeps everyone well inside the 60-minute limit in the Model Aquatic Health Code.
Some small pools rotate every 30 minutes instead: 30 up, 30 down. That is fewer changes, but a longer stretch of watching. The down guard is still on duty, so this is the time for chemical checks, the front desk or a water break, and they must be ready to help in an emergency.
Variations
Other setups
Rotation limits from the CDC Model Aquatic Health Code (6.3.3.1.2). Your facility's rules and certifying agency come first.