The purpose of active rest is to maximize the efficiency of your workout and time. By training with active rest, you allow the blood and oxygen to circulate and help supply the muscles with blood while breaking up and flushing the lactic acid from the worked muscle. It helps with recovery at the cellular level by flushing the cells with nutrient and oxygenated blood.
Active Rest Within a Workout
Without over thinking the term, active rest is a very basic concept. you remain active while giving the muscle worked previously a rest. You can perform a cardiovascular activity between sets of exercises or another exercise such as abdominal crunches. One of my favourite forms of active rest is to jump rope at a modest pace for approx one minute between sets during strength training. For instance, if I was doing a chest workout and performing incline bench press, at the completion of the set of incline bench press, I would immediately jump rope for one minute at a modest pace. This allows me to efficiently use the rest period required for the chest, shoulders and triceps to recover. In addition, because I'm gently moving the muscles of the upper and lower body, I am allowing the blood and oxygen to circulate and transport nutrients while transporting waste products from the fatigued chest muscle. An added bonus is I am also burning more calories during each workout!
Another example is to do crunches between dumbbell rows. While the muscles of the back, rear delts and biceps recover, the abdominals will be worked. You can also do either passive or ballistic stretching between sets.
This theory is similar to the superset. The difference is the active rest ideally will not involve any of the muscles being worked in the previous exercise set. I find doing a cardiovascular activity works best for my body as an active rest. By combining jump rope, jumping jacks, mountain climbers etc. I have found not only does it make the rest periods between sets more productive but also gets me motivated by increasing the heart rate and adrenalin a bit as well.
Give active rest a try for a couple of weeks. This is great to help break through plateaus and motivate you through your workout. Remember, active rest exercises are not to be a full out taxing activity but a "filler" between your work sets.