There are multiple theories. Most have been mentioned here. His artificial hand being made out of Mandalorian Iron etc, or him using the Force.
Just bare in mind that, in 1980, computer techniques weren't as good as they are now. We'll probably never know.
Same goes for the 'who shot first' debate that was caused by a lack of good computer technology as well (Greedo shot first!!)
"I think the Devil ****s Dutchmen."