Have you worked it out yet?
In the bottom of the pot is a pipe which goes almost to the top of the pot. Tea is brewed in a conventional pot and then poured into the upturned Cadogan teapot.
The pot is then inverted and no tea leaks out.
The top of the pipe is now clear of the tea in a bubble of air trapped when the pot was inverted. As tea is poured the bubble increases in size and the tea will never leak.