The Nissen Tube is - to say it shortly - a standard oboe tube (staple), where the cork is removed from the tube, and a plastic part is mounted instead. Rubber rings makes the tube fit firmly in the oboe.
The metal tube is
unchanged, the plastic part is designed to fit standard
brass or nickel silver tubes from all oboe tube makers like Glotin, Pisoni,
Lorée, Nielsen, Rieger, Rigotti, Winfield etc.
The surprising result of this simple procedure is increased evenness of tone and phrasing capabilities. The superb connection with the oboe and the uniform nature of the materials makes oboe and reed one unit, one homogeneous group. To be a little technical: the connection between the reed and the physics of the oboe is stronger, giving the player much better control and output.
An increasing number of oboists are experiencing this - read a few 'reviews'