Our green overlords long ago banned the sale of these in Oz but they are still available on Ebay, Aliexpress & etc (for less than $20). They allow you to use about a third less wood yet produce more heat from your wood stove. Very easy to install, just drill two holes in the pipe and assemble the damper inside it.
We have recently replaced our 40+ year old Maxiheat Logger with the newest model (<$900) and put one in the chimney pipe. The old stove came with one.
You can see in the pics how well they work. For a month we were using the stove without it. Our wood usage has now dropped by nearly half a wheelbarrow full each day and the house is (more consistently) warmer.
The fire is just trickling along nicely but putting out lots of heat because it is not escaping up the chimney.
The stainless steel rod is spring loaded and just slips out and in easily. 
When in place the spring holds the damper in whatever position you chose for it.
