Since my early childhood I've been wondering about the answer to one question about Star Wars:
Why, when fighting with lightsabers, do they use them to parry attacks rather than turning the lightsaber off momentarily?
It seems like they would be able to use this to unbalance their opponent (who is expecting to make contact) then re-activate their lightsaber when their opponent's head is above the handle.
Actually, in the extended universe this technique has seen some use, see Tràkata. Regarding the movies there is also some variation how long it takes for the lightsaber to fully ignite. Sometimes it took quite some time and one might imagine that this delay would be a serious drawback since it gives the opponent quite some time to react.
I am a medieval sword fighting instructor and have won a couple of tournaments so have a different insight than people who probably know the films better but in case you are interested:
Sword (or lightsaber) fights rarely feature aiming at your opponent's sword. If someone aims a shot at your sword you just attack with your sword, their sword flies harmlessly through the air where your sword was as your sword hits them. It is like if you have your fists up and I swing a punch at your fists, just by punching me back you have made me miss whilst I get whacked on the nose.
However, when you are fighting cinematically there are various ways you can fake the fight and one of them is to aim at your opponents weapon because it looks a little more spectacular and you get energetic (and reasonably safe) clashes as the camera can swing back forth tracking you both (I have trained people for stage fighting and you can see many of my group in Gladiator at the beginning).
In the battle scenes it might look like turning off the sabre and letting the opponent miss is a valid tactic but this is only the effect of the cinematic show to make it look better. It is rather like asking why someone does not just move faster when everyone is in slow motion :-)
If they were fighting 'for real' the tactic you describe would probably be quite effective.
Because you'd then at the same time get hit by your opponent's blade that you just let past your defenses?
Besides, you can clearly see that it takes a little time to turn on a light saber - not much, but definitely too much to turn it off and on again within a single swing.
This is actually a recognised saber technique, described in The Jedi Path : A Manual for Students of the Force) as 'Trakata' or 'Passing the Blade'
As to why it's not used more commonly, the answer is that it's a risky (and highly advanced) technique that seems prone to failure.
The Star Wars Wookieepedia page on Trakàta describes a number of occasions on which it has been used. Note that all examples are from non-canon sources such as the EU novels and comics:
Members of the New Jedi Order known to employ this technique include Jedi Master Mara Jade Skywalker. Jedi Master Corran Horn used a variant of this technique during the Yuuzhan Vong War, one notable occurrence being while he was battling Shedao Shai on Ithor. During the battle, Horn and Shai were locked in combat, and Horn deactivated his lightsaber, then reignited it into Shedao Shai, killing the temporary Supreme Commander.
During her duel with Ben Skywalker, Sith apprentice Tahiri Veila used this technique to great effect. Skywalker forced her lightsaber aside, so Tahiri switched her lightsaber off and on so fast Skywalker barely had time to react, allowing her to extend the blade to where his throat had been just moments before
Don't forget that it clearly says (in Phantom Menace if I remember rightly) that the Jedi reflexes come from being able to predict their opponent and react ahead of time. If you tried a trick like this against another Jedi then they would know in advance that it is coming and be able to use it to either still parry/dodge or to kill you while you were yourself unable to block.
There are some very good answers here, but none of them address the fact that such a tactic would fall under the category of deception, which is a weapon of the Sith, not the Jedi. Resorting to such tactics in battle is thus frowned upon by the order.
This however does not stop Jedi from using this or a similar technique when pressed. My favourite example of a move along these lines is from the cinematic movies from SWTOR . In a fight a Jedi is against two sith. The Jedi wields single bladed light saber in one hand and a double in his other hand. He starts with having both blades on the double-bladed saber active in addition to the single blade in his other hand, but later switches one of the two blades off on the double saber. Then while an opponent was behind him he pointed the deactivated end at his enemy and activated it, which impaled the sith.
Also remember lightsabers have no weight in the blade, so unlike a heavy sword, swinging the blade and expecting contact but only hitting air might not have the effect you describe.
If you lock sabers with your opponent and then switch yours off, where do you think your opponent's blade is going to go?
The answer is "into you".
You can dodge, parry, or be struck.
Lightsabers have considerable gyroscopic effect, this makes them hard to handle and requires considerable force experience. Switching them on and off would exacerbate the gyroscopic twist and make it extremely difficult to handle.
Reference : https://starwars.fandom.com/wiki/Tr%C3%A0kata
Philosophically, Tràkata involved practical combat and deception, rather than single minded determination or endless patience. It was rarely used by the Sith, since the power of the Sith came from passion rather than practicality, and neither was it commonly used by the Jedi, because of their unwillingness to rely on deception. The latter may, however, have utilized this technique on occasion, their purpose usually being to exert greater influence over the Force.
Sith don't use this because it breaks the momentum, since most of their power/strength comes from anger.
Jedi dislike this because it is deemed dishonorable.
Jedi dislike this because it is deemed dishonorable - No plaintiff, no judge.