Q. Can we assume that the switched contacts are normally closed, and the connection between (e.g.) pins D and F is made open when a plug is inserted?
A. Hi Grim, thanks for your question. You are right in thinking the switched contacts are normally closed. The input is then directed/diverted to the headphones (or whatever is attached) . In normal (nothing inserted) conditions the signal just passes through from D to F (and E to G). There is a diagram of the connection arrangement at https://www.rapidonline.com/3-5mm-Stereo-PCB-Socket-20-0137#techSpecs