A Tweak is a special type of mod which is able to make changes to the game environment before and during the game load. This type of mod is typically used to make very low-level changes to the game code either because implementing the mod via a ModSystem would be cumbersome/impossible, because using a ModSystem is not desired, or because the mod is a ModSystem itself.

A Tweak can do several things that a regular mod cannot do:

• Alter or add to the command-line arguments which launched the game
• Specify an alternative main class for the game
• Make changes to the game environment before the game is launched (such as injecting extra .jar files into the game or altering system properties)
• Inject into the ClassLoader and provide Class Transformers.

It is the last capability which makes Tweakers so powerful, since hooking into the game's ClassLoader allows almost unlimited control over the game's internal code.