Actually it's pretty simple to do this, but you need to know exactly what to do:
1) First go to your base folder of the Jedi Academy game:
\LucasArts\Star Wars Jedi Knight Jedi Academy\GameData\Base
2) Create there a text file named: Autoexec.cfg open it and then type the following inside:
set g_sabermorerealistic 3
set helpusobi 1
3) Start a new game, as soon as the mission starts press the Quick Save button, then quit the game.
4) Restart the game and don't do anything else! other than opening the console and typing in:
Now you should have dismemberment avaiable after the game loads.
If you already had a saved game on the way, you might want to just skip the "new game part", and only create the Autoexec.cfg file, start the game and then type in console: load quick
or load a saved game via User Interface.
Value 2 of the saber dismemberment will require to swing at opponents in order to get it working, as being stated earlier in this thread.
These things I mentioned here in my post, are only good for the Single Player side of Jedi Academy.