HIIT is an acronym that stands for High Intensity Interval Training. In traditional cardio, such as a treadmill, you maintain a steady heart-rate throughout the workout. In HIIT training you do rounds of higher-intensity exercise, elevating your heart-rate, followed by a period of rest to bring your heart-rate back down. HIIT is highly effective for burning calories both during the workout, and even a "post-workout burn" of up to 2 hours. The HIIT rounds in the TA2 program include 1-minute of high-intensity exercise, followed by 30-seconds of rest.