A 'T-Nozzle' is a nozzle that sits on the end of the launch tube
and once the rocket has got its initial propulsion from the
launch tube, the T-Nozzle engages with the neck of the bottle,
sealing it off and providing a smaller nozzle that will produce
a water thrust that lasts a lot longer than the open neck thrust.
With the initial velocity provided by the full-bottle-neck
diameter launch tube, the rocket can go a lot further on the
smaller T-Nozzle.

The T-Nozzle was invented and developed by Scott Chesnut in
the USA during the 1990's.