Like a normal aux send, an external send taps the signal at a specific point 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 to experiment with external applications or hardware signal processing applications.
Context-clicking on the processor box in a channel strip (at the desired location, pre or post fader) and choosing Add new External Send will show a dialog containing the standard Ardour patchbay to allow to connect the send to the desired destination.
An external send can be removed 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. Dragging it alters the level, Shift-click restores to unity (0dB) gain.
Clicking the small LED in the send display within the processor box turns 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 on the send in the processor box will re-display the patchbay dialog that gives full control over the routing of the send.