MeddleBootstrap provides the ability to use the Meddle mod loader and mods on Curse by using Forge as a stepping stone.
Please Note: You cannot use this mod in conjunction with other Forge mods. Forge is effectively disabled.
The Meddle mod loader is a version-independent loader designed for lower-level tweak class mods. It is often used in conjunction with the DynamicMappings and MeddleAPI mods to provide a more robust mod architecture, which generally follows the latest snapshot versions of Minecraft. You can find more about it here.
Why does it need Forge?
The Curse launcher doesn't allow you to specify a non-Forge mod loader when creating modpacks. There is no mechanism in place to allow Meddle to initialize on its own. MeddleBootstrap was created to achieve a similar goal. The initial phase of FML (Forge's mod loader) brings MeddleBootstrap into the early startup process as a coremod, before Minecraft has started. It then prevents any further initialization of Forge, and allows Meddle to then load its own mods and continue startup instead.
While it is technically possible to run Meddle and Forge at the same time, it is not currently possible to run Forge with anything that requires MeddleAPI. Forge remaps and heavily patches Minecraft's base classes, where as MeddleAPI depends on the original obfuscated classes and operates in reverse by remapping mods instead. Therefore, Forge can't be allowed to initialize. This may change in the future.
What's different between this and standard Meddle?
MeddleBootstrap carries out a second purpose as well, which is to force Meddle to use the Forge directory structure instead of Meddle's default. This allows Meddle mods to be posted on Curse, and the launcher will put them in folders that Meddle can find. The same is true for config files in the case of modpacks. So basically, your Meddle mods go in the same folder that MeddleBootstrap does, which is simply "mods".
There is one consequence of using this loading technique on Curse. While Meddle, DynamicMappings, and MeddleAPI have moved forward and are capable of supporting even the newest 1.10 snapshots, using MeddleBootstrap on Curse will limit you to the latest version of Minecraft that Forge supports (1.9.4 at the moment). Theoretically you could use Forge for Minecraft 1.9 in 1.10 snapshots, and that would be sufficient to initialize Meddle at such an early stage. But in this case, the Curse launcher overrides the specified version of Minecraft in your modpack's manifest.json with the version of Minecraft that Forge requires. You can at least use appropriate Meddle mods in 1.9.4 through Curse for the time being.
If you want to play on the latest Minecraft snapshots, you'll have to use the standard installation process described on the Meddle page.
Can I use this on other launchers?
Absolutely, though it might not be necessary, and it's recommended to use Meddle directly when possible.
If the default version of MeddleBootstrap doesn't work in your circumstance, the source code is available.