It works best on enh shamans due to Windfury.
You'll need 2 kinds of weapons (2x 2h or 1x 2h and 2x 1h)
then make a macro with
/equip <weapon name>
/equip <weapon name>
and if you have 3 weapons (2x 1h & 1x 2h)
then add the last weapon there too. So, equip the weapon that is the last one you wrote on the macro. Now, target a hostile person/mob, put auto-attack on, and then start spamming the macro, or time it that when you hit, press the macro. As you will get a new swing with the other weapon(s) you change to, making it 2x damageWorks on wowscape at least.