This plugin is pretty simple.

There is a mod which allows you to view the health of all mobs and players.

Giving a unfair advantage in a pvp fight, survival etc.

Other hack clients provide this feature as well.

What this plugin does, is when the server sends the health to the client.
The plugin catches the health. Then messes it up to a incorrect value.

To use this, you need ProtocolLib and Lib's Disguises .

However!

If you are using scoreboards to display players health.
Then its likely to conflict!