While it sounds like a contradiction, its possibly one of the most powerful weapons in your training arsenal and its something which I use myself and with my Clients on a daily basis.Active Rest means your using your time productively between sets. So in stead of sitting down, wasting time and letting your heart rate cool down between sets you can focus on a different exercise/muscle group while the other muscle group is resting, then repeat.

For instance if you are training Chest (i.e. Flat Bench Press) and you have 4 sets to do you, you can train an entirely different muscle group such as Abdominals (sit ups, crunches etc.) between your Chest sets and then returning to Bench Press and repeating this process until 4 sets of each are complete. The advantage of this type of training is that your not wasting time, you will have numerous sets for i.e. Abdominals by the end of your session and your workouts are shorter and more productive. Whatever muscle groups you choose to train with Active Rest just make sure they don't interfere with the performance of the main muscle group. For example if you are training Chest but using Active Rest with Triceps (i.e. close grip bench press) then you will affect the performance of the main Bench Press exercise as the Triceps are involved during the bench press extension phase so it will decrease your strength, maximum lift and performance on your main exercise leading to diminished gains. 

Popular muscle groups to train together using Active Rest are - Shoulders & Biceps/Abs, Chest & Biceps/Abs, Legs & Shoulders, Back & Triceps.Using the Active Rest method will increase your intensity and decrease your time in the gym meaning more muscle stimulation and more recovery which means more muscle growth, strength and progress/gains. Give it a try next time your in the Gym and remember - go hard or go home! 

- APG