Empty City Ploy
It sounds like the empty city ploy is about presenting a group of stones which appears to have been deserted, and when they are attacked you expend your energy somewhere else. For example, you have a large group, perhaps you should sacrifice it. I have had a number of games in which, I have sacrificed or given up a very large and hopeless group, going on to win the game by a good margin. Basically, let your opponent attack windmills if he wants to, while you work on something more devious.
Another example is from a more recent game, my opponent attacked a small group of stones and expended several moves doing so -- thereby allowing my troops to attack elsewhere. The net effect was that yes, he captured that small, difficult-to-work-with group, but I made headway all over the board.
If my memory serves me well, historically, the Empty City Ploy describes a situation where a beseiged army desires retreat. For three nights the defenders would rouse the attackers in the middle of the night with loud sounds, like that of a sudden counterattack. Each time there turned out to be no threat. But on the fourth day, when the invaders finally take the castle, they found nobody there - the defenders had already slipped underneath their tired eyes.
The other kind of empty city ploy is to leave a few troops in posession of a stronghold or camp, when the camp is attacked the attackers find that no-one is there throwing them into a state of confusion at this point the army which should have been in the camp attacks usually by means of a flanking technique or from a concealed location. In Go this could translate as a group of stones played lightly which look tempting to the opponent but when eh attempts to capture them you can spring a trap or similar device.