first...you got a point there that when a lightsaber falls it seems to automatically shut down...maybe it's like shocks with an airbag...If the hilt is hit hard the lightsaber shuts down...lol second. There are several Jedi combat moves. In defence it's better to hold it like samoerai's do, cause some fighting style is based on that. Vader is very powerful with a blade and Sith do care less for defence so he has his own style. Mace Windu also sometimes fights with one hand. Depends on the skill, like also with humans. Another reason is that lightsabers are very heavy in the classics to hold, so you'll need the strenght of both hands to keep it up. In the prequels the lightsabers don't weigh as much as in the classics, which explains why they are more dynamic cause the lightsaber is not so heavy to hold. The ironic thing is that with the hilts it's the opposite...the hilts of the prequels are a lot more heavy than the hilts of the classics I know that David Prowse wasn't always the one fighting...I have a picture of Bob Anderson fighting in Vader's armor...you can see that his boots are different cause he doesn't have the same height as David Prowse.