TUT stands for 'time under tension' and is defined by the duration which you are engaged in performing a set.  If you are able to increase your TUT, you are able to increase the blood, oxygen, and hormones going to the muscle.  There are a few ways to achieve this - the first being an increase in the rep range that you are performing.  If you are normally performing say 12 reps by increasing that number to say 18 or 20, you are increasing the amount of time that the muscle is under tension.  An alternate way to increase your TUT is to slow down the speed that you are performing the repititions.  Studies suggest that an ideal time for TUT is 40 to 70 seconds.  I would suggest to all of you out there to give it a try and see how it goes for you.  If nothing else, it will be a change in your training (muscle confusion principle), thereby shocking the muscles into growth.