Hagmann valve

The Hagmann Valve works similarly to any other open-wrapped trombone valve configuration. Pulling the "Trigger" causes the spindle to turn, guiding the air through a different set of tubes. It is different from the rotory valve in that the angles at which the air flows (60%, or 66% in bass trombones) is dramatically less, and preserves tone. While the Hagmann Valve does not achieve as low an angle as the Thayer Valve (which can be as low as 20%), it is much easier to maintain, and tone differences are nearly inaudible.

Sources

http://trombone.ch/