HIIT is when you alternate between short bursts of high intensity exercises followed by differing lengths of low intensity recovery periods. The higher intensity period creates an oxygen demand in muscles that aerobic respiration cannot fulfill fast enough, so the body opts for anaerobic respiration instead. During recovery periods the body switches back to aerobic burning of calories but is forced to switch back again to anaerobic breakdown during the following high intensity phase. This creates a large enough energy and oxygen deficit that the body spends about 24 to 38 hours recovering from, after the workout has been completed. This ensures a prolonged afterburn effect, causing the body to burn fat long after the workout is over.