Good question. It was absurdly lax of Vader to allow that opening on an otherwise beaten opponent.
It has been suggested that Vader's armour is sabre resistant... but I don't buy that, especially when words like 'cortosis' start being spread around.
Actually, if you look at the move in slow-mo, it wouldn't have sliced him open, the angle wasn't like that, but it should have fair skinned his arm!
I actually think the blow connected more in shooting than it was meant to; they weren't as meticiulous then as they are now. I really think it was only meant to be a very glancing blow.
Or perhaps Luke didn't expect to hit vader and therefor allready had started to reverse the blade.
If u ever train fighting/sword fighting u may experience the same. U start a blow but u asume that the other one is going to manage to deflect it, so u don't put all your strength in it and u are allready thinking about your next move. So when u suddenly manages to get past someone guard when not expecting to, the blow gets very weak and u've lost your chance. Got it?
Well, Vader was being lax with Luke, he wasn't on his toes. We all know Vader is arrogant in his abilities, and it's been a while since anyone remotely challenged him physically. He was surprised.
And, they didn't practice their choreography half as much as nowadays, so that shot was probably clumisiness on Mr. Hamills part, being a bit too rough.
Another explanation is that Vaders suit is made from a very strong material, stronger than most metals, but not invincible. Like we saw how a lightsaber actually has resistance when put into metal, as seen in Ep1 when Qui Gonn was opening those doors. Maybe Vaders suit is made of uber-tough metal, and therefore, a quick, glancing slice like Luke's swipe would have only caused a small nick.
Also, in Dooku vs Obi-Wan, Dooku cut down Obi-Wan in a similar fashion before going for the final blow.
I always believed that Darth Vader's armor is made of a nearly indestructable material, which would help protect him. He is also the Star Wars jesus, which means he could probably make new force powers or use the force in ways never thought of before.
And he felt the pain from it, causing him to get extremely angry and remove Luke's arm.
well in ESB, Han solo shoots a blaster against Vader which he deflects with the palm of his hand. if he used the force to do that, i dont know. But what i know is that Cortosis Ore
Dark ArmorDark armor is nearly always created to meet the specific needs of the wearer. The material and design varies from wearer to wearer. Some suits are crude and heavy, while others are elegant and light. A few seem almost decorative. Almost all of them are cortosis treated during their construction to achieve certain affects desired by the wearer, from additional protection against blasters and lightsabers, to extra resistance against certain forms of attack.
Some examples of Dark armor in the past were worn by the ancient Sith warrior, Larad Noon (time period: Sith Wars). This heavily padded armorsuit incorporated cortosis weaves that wreaked havoc with any lightsaber audacious enough to slice into it.
Dark armor is never simply found; those who wear it, almost always take it with them to the grave.