Wind shutoff valve
An organ control that shuts off the wind supply to an entire division.
What it means
The wind shutoff valve is a device on the organ that lets the wind supply to an entire division — such as the Great or the Swell — be shut off or released as a whole, regardless of which individual stops are drawn.
It sits in the main wind trunk feeding that division, acting like a central switch for that division's whole wind supply.
Musical use
In practice, the shutoff valve makes it easier to quickly silence or reactivate a complete division, for instance to avoid unwanted sounds during a stop change while stops are already prepared but the wind is still shut off.
It can also be used during cleaning or tuning work on the organ to disconnect a division from the wind system in a targeted way, without having to shut down the whole instrument.
The wind shutoff valve should be distinguished from an ordinary stop knob, which switches only a single stop, and from the swell, which continuously varies a division's volume rather than cutting off the wind supply entirely.
Performance practice
As an organist, you usually don't operate the shutoff valve yourself, but you should know that an apparently silent stop is sometimes simply due to that division's shutoff valve still being closed.