By doing two exercises that target muscle groups that are far away from each other (for example, quads and shoulders) in rapid succession, the body has to quickly adjust blood flow to the working muscles on different "ends" of the body. This constant need to adjust blood flow makes the heart and vascular system work extra hard. This is great for mobilizing fat and cardiovascular health.
Stick to slightly lower reps so you can handle fairly heavy weights while still getting the metabolic boost and cardio effects. Shoot for 6 to 8 reps per set, but you can go up to 10-12 reps on some exercises. Pair two exercises together and rest roughly 20-30 seconds between both. Example:
Leg press, 10 reps
Rest 30 seconds
Dumbbell shoulder press, 8 reps
Rest 30 seconds
Leg press, 10 reps
Rest 30 seconds
Dumbbell shoulder press, 8 reps
Rest 30 seconds
Leg press, 10 reps
Rest 30 seconds
Dumbbell shoulder press, 8 reps
The Rules
You can use any exercises you want, but for best results apply these rules:
Include one upper body and one lower body exercise per pair.
Try to avoid exercises with a strong postural component, like squats, deadlifts, and bentover rows. If you're not used to high density training, you might find it hard to maintain a proper lifting posture when you get tired.
You can use isolation exercises, but do them near the end of a workout or pair them with a very demanding lift.
Start at 3 pairings per workout until you become more efficient at high density work, then consider moving up to 4.
Control the eccentric or negative portion of the movements, even going fairly slowly.