In order to enable the Chainsaw Beam, the Ice Beam must be disabled, while the others should be enabled. In order to bypass the fact that the Spazer and Plasma Beams cannot be enabled together, the Spazer must be enabled. You must then go to the Boots section of the Inventory screen and press left and A at the same time; if done correctly, then the Plasma Beam will be enabled alongside the Spazer, the former having a subcategory with only one option in it; VAR. Firing a charged shot or attempting to use a Charged Chainsaw Beam and special Power Bomb attack will crash the game.