For carrying a small child on a Brompton. The IT Chair clamps to the seatpost at the back and hooks under the main hinge in the centre of the frame. The child has a pair of footrests and can hold onto the handlebars. It releases by undoing a standard Brompton clamp nut which clamps around the seatpost. Then you can just unhook the front and swing the IT Chair to one side of the frame when you fold the bike.
The newer version of the IT Chair is made of aluminium and has fixed foot pegs.
You can add a saddle with a clip.from the options list or use any saddle with a clip. Thanks to Documentally for the pictures.