The idea is very simple. Say you have a hand of village - armory - conspirator - copper - estate
1) Play the village
2) Play the armory, gaining a conspirator on top of your deck
3) Play the conspirator, drawing the other conspirator
4) Profit
If you don't have enough village, simply gain a village on top of your deck instead of conspirator.
One of the funniest uses for armory.