One of the earliest journal articles using the term HIIT and studying the positive effects was from a sport scientist in Canada, Angelo Tremblay back in 1994 (see references). Tremblay showed amongst other things a greater fat loss for HIIT subjects compared with those following an endurance training or low intensity workouts. This was inspite of the actual exercise burning fewer calories compared with the lower intensity workout.
A typical interval training session can be summarised as short burts of vigorous exercise followed by typically shorter periods of rest repeated a number of times. The typical bursts of power last from 20-30 seconds up to a couple of minutes with recovery times between ranging from 10 seconds again up to a couple of minutes.
Why are Tabata Intervals Different?
The difference with the tabata intervals is the short length of the intervals. Typically intervals were between 30 seconds to a minute in something like a circuit class.
The tabata intervals are 20 seconds of power followed by 10 second recovery or low intensity rest.
Metabolism. 1994 Jul;43(7):814-8.
Impact of exercise intensity on body fatness and skeletal muscle metabolism.
Tremblay A, Simoneau JA, Bouchard C.
Pysical Activity Sciences Laboratory, Laval University, Ste-Foy, Quebec, Canada.