An insert effect sits directly on an audio track, affecting all parts of the audio signal. If you insert a compressor onto an audio track, for example, the effect will apply its full level of compression to the signal. A send effect, on the other hand, sits on a specialized return track, allowing you to control the degree to which the effect applies to audio tracks. To apply the send affect to an audio track, turn up the track's Send control. The higher you turn the Send control, the more the audio track mixes with the return track. If you set the Send control to 50 percent, for example, you'll hear half unaffected audio and half affected audio.
Use an insert effect when you want to apply the full force of an audio effect to an audio track; use a send for more subtle effects. Try adding a delay effect to a return track, then slowly increase the audio track's Send function until the two audio signals blend together without one overpowering the other. You can also use a send effect to apply effects consistently to several audio tracks. For example, you can put a reverb effect on a return track, then send all of the audio tracks in the project to the return track. Running all of the audio through the same effect creates a much more natural-sounding ambiance than putting a separate insert effect on each track. Using one send effect for multiple tracks in a digital audio workstation also greatly reduces CPU load.
Certain types of effects work better as insert effects; other types function best as send effects. Reverbs, delays, phasers and flangers, for example, sound best when you use them as sends, while compressors and EQs usually work better as insert effects. Experiment, however, with using different types of effects as sends and returns: try, for example, placing both a compressor and a delay effect on a return track, sending an audio track to the return and then sidechaining the compressor to the audio track. This chain produces a delay effect that activates only when the audio track falls silent.
Even if you're using a certain effect as an insert, you may be able to control the degree to which the effect applies as if it were a send effect. Look for a "Wet/Dry" or "Dry/Wet" control on the effect; this control determines the degree to which the "dry," or unaffected, audio mixes with the "wet," or affected, audio. Turn the control towards "Dry" to decrease the impact of the effect; turn it towards "Wet" to apply more of the effect.