Don't let what people say about the plasma cutter upgrades put you down. Upgrade it, trust me. The fire might not make much of a difference, but the plasma cutter is still one of the best weapons in the game. Upgrading that and the contact beam is the best option.
Your question has already be answered. As JBlanton already said, it does "exactly what it says." When in the Bench, there is a section at the bottom that tells you in detail what stats you will be gaining for inserting which node. There, you can view a description of the weapon's Special.