The synth is a pretty powerful tool. You can think of it as a way of creating complex imagery (i.e. imagery that is made up of more than one visual element). It works especially well with abstract-like images.
I'd say look through some of the beam-related cues. You can see which ones are made using the Synth because each cue has an icon that indicates what the cue contains. The icon for synth cues looks like two wedding rings intertwined.
If you right-click and say "Edit synth-image", then you will be able to see how the cue is made up, and see how this works.
We did add a "mute" capability to individual image synthesizer elements. We believed that this was a good idea you had, and we thank you for the suggestion!