Like a normal aux send, an external send taps the signal at a specific within a channel strip, but delivers it to an external application or piece of hardware rather than an Ardour bus. By itself, an external send has no effect whatsoever on the audio signals within Ardour—it is a one-way signal routing that leaves all existing signal processing just as it was.
Most people will not have much use for this, but it can be useful if you want to experiment with external applications or hardware signal processing applications.
Context-click on the processor box in a channel strip (at the desired location, pre or post fader) and choose Add new External Send. A dialog will appear containing the standard Ardour patchbay to allow you to connect the send to the desired destination.
You can remove an external send in several ways:
Just below the send in the processor box is a small fader that can be used like all other faders in Ardour to control the gain applied to the signal delivered by the send. Drag it to alter the level, Shift-click to restore to unity (0dB) gain.
Click the small "LED" in the send display within the processor box to turn it on and off. When turned off, silence will be delivered to the send. When turned on, the signal within the channel strip will be delivered.
Double-clicking or Edit-clicking on the send in the processor box will redisplay the patchbay dialog that allows you full control over the routing of the send.