Manually Process Rules

What you do is install the addon using an installer script that's supplied with it. Then you simply go into Mail's Rules Editor and add some rules in the style of the example below. The Act-On part of the description links the rule with the addon.

These new rules are for manual use. When you're reviewing a new message and decide to apply one of the manual rules: press "~" and up pops the cool and sexy looking overlay below; I've just installed it so I've only set up two rules so far, but you get the idea...

Then you just hit the key shown in the box (in this case, "a" or "r") to apply the rule. If you do nothing the overlay fades away after five seconds.