A classic HIIT workout consists of alternating periods of work and rest.
During the 'on' periods you'd work at your maximum effort and really go
hell for leather repping out the exercise. Then, when the whistle or
timer blows for the recovery period you'd stop and try to catch your
breath and bring your heart rate back down.
The 5 basic principles of a good HIIT session are:
Exercise duration - each exercise should
only last up to 60 seconds.
Recovery - this is where you let your
heart rate drop back towards its resting rate.
Intensity - reaching up to 85-90% of your
maximum heart rate during the 'work' periods to get the most out of
the session.
Work:Rest ratio - will depend on the
workout and should vary.
Frequency - if your goal is fat loss, 3-5
HIIT sessions a week is a good number to aim for.