When the first Death Star was destroyed, the Emperor punished Darth Vader by removing his arm (according to the novel "Heir to the Empire"). This was originally said to explain Vader's mechanical arm, but it was later revealed that all of Vader's limbs were mechanical after a near-fatal duel with Kenobi (technical retcon is he removed a prosthetic and Vader got a new one). But *how* did he do it? 

When that info was first revealed, the Emperor wasn't known to wield a lightsaber. Of course *now* we know he did, but back then we assumed he didn't have one. With that retcon in mind, still by 0 ABY, Palpatine didn't have a lightsaber "anymore" and wouldn't awkwardly get up, activate a lightsaber and chop off Vader's arm. It's not his style. I imagine him forming an "energy blade" out of pure Force energy or something to cut off the limb. Does my theory have any credence with canon, or is there another amputation method?