HIIT workouts are composed of short periods of intense work, then a period of rest or active recovery. "Intensity" is the most important part of HIIT participants should be working near or at peak effort during the intervals, then backing off during the breaks. Since this high level of effort can pump up the heart rate, some trainees use HIIT protocols as the cardio component of their exercise routines instead of low intensity steady state training, (LISS) like long runs or cycling.
HIIT requires you to be explosive and intense during your work period. Some researchers have found that HIIT increases metabolism for hours after exercise even more than jogging and weight training. This is known as excess post-exercise oxygen consumption (EPOC, informally called afterburn), a measurably increased rate of oxygen intake following strenuous activity.