Channel insert points
The insert point of the channel is used to connect a compressor, noise gate or external equalizer - never a reverb.
A unit connected to the insert point will only affect the signal on that channel.
The insert send sends signal to the external unit; the insert return accepts the output of the external unit.
The insert point of many consoles is in the form of a stereo jack socket (3-pole jack). The tip connection is the insert send and the ring connection is the return. On some consoles, the connections are reversed. A Y-cord is required which has one stereo jack connected to two mono jacks for input and output.
When there is no jack in the socket, a switch contact passess the signal straight on to the rest of the circuitry.
There is another use for the insert point besides sending the signal through an external unit. The insert send can be used by itself as an extra output from the console for the signal on that channel only. You would normally only do this if you were already using every other output.
To use the insert point in 'send only' mode, a special cable must be made that links the tip and ring of the jack so that the signal is passed through, as well as providing the extra output.
Usually, the channel insert point comes directly after the preamp stage of the channel. In some consoles it comes after the equalizer. This can be better for compression, but it plays havoc with noise gate threshold settings.