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.

Forge ModLoader (FML) has a capability for a mod to elect to become a “core mod” which sounds very similar to the features just described for Tweakers above, and this is no coincidence. This is because FML is a Tweak itself and chooses to delegate its power to “core mods” which require it, any Tweak can do this and LiteLoader does so as well, although it doesn't make a distinction between “core mods” and “regular mods” and simply lets any mod supply Class Transformers if it wants to.

In a nutshell, a “core mod” accepts delegated power from a Tweaker without becoming a Tweak itself, this makes it dependent on the first Tweaker and not on the Tweak System directly.

Whilst it is incredibly powerful, the Tweak System itself is actually incredibly simple. It leverages a library called LaunchWrapper originally written by cpw for Mojang in order to allow them to run old versions of Minecraft inside the new launcher. To use Tweaks, LaunchWrapper is simply added to the game environment and the game's main class is replaced with LaunchWrapper's main class. Tweak classes can then simply be specified on the command line using –tweakClass arguments.

Since the original aim of LaunchWrapper was to allow old versions of the game to be run in the new launcher, there was no provision made to allow more than one Tweak to be specified at a time. In version 1.7 of LaunchWrapper, cpw added the ability to specify multiple tweaks, but prior to that a contract of 'cascading' tweakers was agreed between cpw and myself to allow whichever tweaker was loaded first to load the other.

This is why in newer versions of Minecraft you can simply specify multiple –tweakClass arguments but before that it was necessary to specify the first tweak using –tweakClass and additional tweaks using –cascadedTweaks