A lightsaber duel between two Jedi consists of several different things all interacting at once in ways which would be beyond comprehension or utilization by non-force sensitives. The first and most obvious element is the physical act of dueling: the technique and physical exertion that goes into any sort of swordfight. This is honed for years by the Jedi, who are trained from the earliest years of childhood in more than 7 different overarching fighting styles, each completely different and focusing on different skills, and each with hundreds if not thousands of other skill factors within it. A Jedi in her prime has trained her agility, her physical strength and endurance, her stamina and control, even her breathing and heart rate, to be at their optimum level, and to be able to utilize her skills any time she needs them requires immense skill and a lifetime of regimented practice.
The second element is emotional control: there is a reason Jedi are taught to separate themselves from their emotions. In a life or death situation, such as a duel with another talented duelist, you cannot let your emotions get the better of you. If you give into your fear, you will be cut down. If you give in to your empathy, you will be weak and hesitate to throw the final necessary blow. If you allow your emotions to bubble out of control, you will lose control of yourself and, ultimately, your life. This is the strong difference between the Jedi and the Sith: the Sith, unlike the Jedi, do not avoid emotion. They embrace it, believing that it will give them power. And through embracing hatred and anger, by idolizing attachment, you can become a master fighter. However, such emotional surrender will make one more apt to risky and dangerous decisions. They might try maneuvers that will almost certainly fail, simply because they are blinded by a desperate need to destroy their opponent. It also destroys any hope of a peaceful solution, and makes negotiation, teamwork, and planning much more difficult.
The third element, and the hardest to grasp, is the Force. 999
Because this is such a broad topic, I will break it into three subcategories, the first of which being force assisted moves. With the Force, a Jedi is stronger than the average duelist, faster than the average duelist, and overall more powerful. He can make greater jumps, do faster and more demanding maneuvers, and use his environment as a weapon in ways which non-Force users cannot. This is the most readily visible element of the three Force considerations. The second is the use of the Force itself as a weapon. There are many fights in which you see Jedi apparently slip up: they leave something unguarded, they ignore an opening, they do something which looks cools but leaves them completely open (for example, spinning to gain power or jumping over and landing with their back to their opponent), they dodge a move which wouldn’t have harmed them, or they go halfway through an attack and then back off, even when the outcome would have been victory. These are easily explained if you consider that the Jedi are consciously using the force as a weapon (though sometimes they just slip up, of course). Often, if something is left unguarded, it is a feint, designed to make an opponent commit his entire offensive into one killing blow to a place which is defended by the Force, allowing the Jedi fighter to catch him off-guard and strike at him when his concentration and stance are broken. Similarly, when the Jedi spins around, she can be assured that a quick use of the Force will allow her momentary protection to gain back up her speed and re-adjust her stance and/or style. If a Jedi halts an attack halfway through, he has realized that the attack would be useless, and may well have fallen for a feint. The third category under the Force consideration is the “will of the force.” Often something happens simply because, for some unexplainable reason, it is the will of the Force. Often Jedi, especially those lacking in training, can rely on the Force to get them through a fight, even with a far more skilled opponent. Just as relying on the force can allow one to sense feeling and intentions, or to block laser bolts on instinct, so too can it be used during a duel to anticipate an opponent’s next move or strategy. Thus, half the battle is invisible, done entirely on a level which we cannot sense but which the Jedi and her opponent can fully understand, feel, and counter.
This is why <PERSON> appears so weak in his skills in the original trilogy: he lacks the years of arduous training, the knowledge and practice of legitimate lightsaber techniques, and the skill and experience to be able to consciously and in split-second decisions use the force during his duels. 999