Time under tension is the amount of time the muscle is under stress during a particular set. The greater the stress and the more time under tension there is the more muscle growth you can expect. You can greatly increase time under tension with various tempos, supersets, and drop sets. All 3 are different so let me explain each. A tempo is the particular speed at which each set and rep is performed. The most difficult, but most effective for muscle gain and time under tension would be the 2-2-2 or 2-2-4 tempo. How this works is you will focus on 2 seconds on the concentric motion, you will hold the contraction on the muscle being trained for 2 seconds, and then the eccentric motion (lowering of the weight will be for 4 grueling seconds). The way a superset works is to perform 2 exercises at the same time without allowing any rest until you've completed a set for both exercises. For example, if you're training your biceps you would do a set of alternating dumbbell curls, and immediately without taking any rest would go over to a cable machine and perform a set of rope hammer curls. Then you would take a rest for about 60 seconds and do it again. So, as you can see the biceps would be under tension for a long period of time thus causing them to grow and expand. Scott has demonstrated the fundamentals of drop sets numerous times in his training videos so I wont go into detail there as you should all be familiar with it. Always use a weight that you can control. You want those last couple of reps to take a lot of pushing to do without breaking form. Keep the reps around 8-12. Just always remember that you need to feed your muscles quality nutrition if you expect them to react and grow from this type of training.