For some time now we have been acquainted with the humble target dummy: A device that, at first glance seems unassuming, and straight forward in purpose. Many members soon discovered it had intriguing properties and capabilities. This said, I think the dummy deserves it's own tutorial/research page. We have only been exposed to it for roughly eight months now, and it's very well possible we could find many more uses for the little guys down the road. (Especially with 1.3.1 lurking off in the future.) This thread will contain what we currently know about this fascinating item, and can serve as a thread for new Dummy observations and discoveries. For the sake of reference, I personally will start referring to the two parts of a Target dummy as the "Dummy Ghost" (The invisible "entity" that is capable of being moved from it's original position), and the "Dummy Host"(The furniture piece that remains immobile no matter the stimulation). 
Target dummies are background furniture items, crafted with 20 blocks of wood and 50 blocks of hay at a Sawmill. At first glance, their main purpose appears to be for the use of testing the players damage output. However, observant players soon found that they are actually capable of much more! There are two key components at work in a Target Dummy...
This is the actual furniture item the player is placing. It can't be moved, and it acts like pretty much any other background furniture piece (Save for performing an animation when struck) The Ghost is directly tied to it. Only one ghost can be tied to a single respective Host at any given time. If the Host is destroyed, the Ghost will despawn.
The Ghost
The Dummy Ghost is the Npc that spawns when the player places the Target dummy. This is the entity that responds to attacks, and displays damage. In our case, this is the component, while immobile in most cases, that can be moved around with proper techniques, and utilized in mechanisms.
-The ghost has a "despawn" range of 10k tiles. This is well above the size of even a Large Terraria world, so the likely hood of despawning them offscreen is unlikely. That said, the main ways a ghost can be despawned is either through the player dieing, exiting the game, or removing it -Ghosts "spawn in" when the player is within a range of 100 tiles from them. -The ghost is immortal. -The ghost is "movable". It can be teleported, and moved by hoiks. -The ghost is affected by gravity, meaning it can fall if teleported over empty space -It's been observed that only 100 ghosts can be active at one time (observed by ZeroGravitas). They are still versatile nevertheless, since multiple dummies can be in one area (contrary to statue spawned mobs, which can only have 3 critters/enemies out at one time) -The ghost is affected by liquids. (Water, Honey, and Lava) Honey, in particular, can be used to slow down the descent of a falling dummy ghost. (This is useful in delaying engines. The more the honey present in the machine, the longer the delay.)
The dummies have an interesting attribute: They spawn an invisible "Npc" when they are placed to represent themselves. What's more, this Npc can actually be manipulated in surprising ways. Here are some of the key attributes of this interesting item:
-One of the primary uses of the Dummy, is in batteries and engines. Due to the fact it's easy to obtain, and due to it's unique attributes, it's one of the best candidates for them around. (The average player can mainly start using them after defeating a mechanical boss)
-Target dummies can also function as components for hoiktronics.
-they can trigger invulnerability ticks from mounts (slime and unicorn mounts, specifically. Thanks Bahamut2001)
(Hopefully this section can be expanded over time if any more apparent uses for these things are discovered)
-Due to the fact Target dummies are immortal, different measures will have to be taken if the player wants to reset the machine. (As traps, weapons, and minions won't do the job)
Since dying would probably be too inconvenient, exiting and re-entering the world could be one option. The player could also just break the host if they wanted to reset the machine, as the Ghost is tied to it, and will despawn if it's respective host is destroyed. The player could just break it themselves, or actuate the blocks underneath it.
-Dummy Ghosts can be moved via teleporters, and hoiks. The player can also use gravity to drop them too.
-It can be handy if the player puts the dummy in mid-air using the teleporter trick, as this will cause the Ghost to fall straight away.
Conclusion
In conclusion, I hope some of the rest of you can help me expand this thread over time, as I might have missed some details XD.. (Custom visuals created by me with Gimp. Ghost dummy icon created by me also)
*No dummies were harmed in the making of this thread* Side Notes:
Since Dummy Ghosts don't actually have a sprite in game, you could check out the Mechmod. One of the mod's many features happens to include a sprite for the ghost, allowing you to see where it's going. You can also use the mod to get rid of wayward dummies. That said, it has much more functionality than that, so go check it out!
NOTE: There's a strange bug involving dummy ghosts being in a Terraria world without being bound to a Host. (Dubbed the "Wayward Dummy" glitch) The bug is usually caused by attempting to remove a dummy's sprite in TEdit, however some users have documented it occurring in worlds where they've never used Target dummies at all/don't even use TEdit. Please refer to this thread if you ever run up on this bug.
