Green johanna

I'm researching compost bins. 

The Green Johanna is classed as a hot bin, which the Council is offering at £50, a very good price,  but having bored myself silly listening to a video about assembly and what can go in it, I can see its quite sturdy but why is it called a hot composter? I can see no extra insulation and it seems just a plastic shell with a base and lid. How does the GJ get to a higher temp than any others? I can purchase a winter jacket (currently out of stock), which makes sense but without that it seems to me that it is just a normal plastic compost bin.

Anybody able to educate me? At the moment I think I'm sticking to what I've got, which is a square plastic black one with an improvised lid - the lid blew off in the recent  gales which prompted this whole process

Another thing, how robust is the winter jacket, I figure if rats can chew through the plastic on my black one, they'll  dine out on the insulating jacket thinking they can smell food inside.

When I looked into hot composters, I also wondered why the green johanna was classed as one. Never did work it out. All I can think is it’s winter jacket, but I’m only guessing. It might be worth a go if you’re council are offering them for £50. Like you say that’s a very good price.