Martial arts teachers often fail to explain this concept of relaxed-tension accurately. Much of the difficulty is a result of mistranslating the word "relax." Being relaxed is a wonderful thing when you're stressed and rattled. But when you want to exert strength, power, speed, agility, explosiveness, as well as flexibility, relaxation alone will get you nowhere. You must have a certain degree of tension within your relaxation - or a fair percentage of relaxation within your tension.
Hundred percent relaxation is an erroneous objective. Eighty percent relaxation and twenty percent tension is a good place to start, if you want to think in terms of numbers.
But the truth is you will figure out how much tension and how much relaxation is ideal for you when you follow the exercises Systema lays out for you when you train.
As you train, you don't need to think much about how much of one or the other. The answers will come to you. Your brain and body will tell you how much tension you need to use. Trust the process and you will see.