The fundamental principles of High Intensity Training (HIT) are that exercise should be brief, infrequent, and intense. Exercises are performed with a high level of effort, or intensity, where it is thought that it will stimulate the body to produce an increase in muscular strength and size. Advocates of HIT believe that this method is superior for strength and size building to most other methods which, for example, may stress lower weights with larger volume (sets x reps).
As strength increases, HIT techniques will have the weight/resistance increased progressively where it is thought that it will provide the muscles with adequate overload to stimulate further improvements. There is an inverse relationship between how intensely and how long one can exercise. As a result, high intensity workouts are generally kept brief. After a High Intensity workout, as with any workout, the body requires time to recover and produce the responses stimulated during the workout, so there is more emphasis on rest and recovery in the HIT philosophy than in most other weight training methods. In any workout, not just HIT, training schedules should allow adequate time between workouts for recovery (and adaptation).
While many typical HIT programs comprise a single-set per exercise, tri-weekly, full-body workout, many variations exist in specific recommendations of set and exercise number, workout routines, volume and frequency of training. The common thread is an emphasis on a high level of effort, relatively brief and infrequent (i.e. not daily) training, and the cadence of a lift, which will be very slow compared to a non-HIT weight training routine.
Most HIT advocates stress the use of controlled lifting speeds and strict form, with special attention paid to avoiding any bouncing, jerking, or yanking of the weight or machine movement arm during exercise. Technical HIT advice varies from lifting the weights smoothly and at a natural pace, to timing the lifts, peaking at hold and descent. In extreme cases, it may take up to 30 seconds to complete a single repetition. While high intensity training is strongly associated with Nautilus exercise equipment, advocates vary in their equipment recommendations.
Also emphasized when near exhaustion in order to further exhaust the muscle or muscles exercised: doing static holds for periods of time, and negative reps (lowering the weight). This will stimulate further growth and strength because muscles are weakest in positive/contracting movements (sometimes referred to as first stage failure of a muscle). Although you may not be able to lift a weight for another rep you will almost certainly be able to hold it statically for a further period (second stage of failure) and finally lower a weight at a slow controlled speed (third stage of failure). Until all three (lifting, holding and lowering) parts of an exercise can no longer be completed in a controlled manner a muscle cannot be considered thoroughly exhausted/exercised.