Learn something new every day.
When using the color selector and the wand, if you click and drag(*) (instead of just clicking) this changes the threshold and shows you instantly what you have selected.
This has been around for a a while (at least since 2.8). Not really a secret since it is described in the doc, but who reads that
To experiment, create an image, fill it with Plasma (Filters>Render>Cloud>Plasma), star one of the two tools and click+drag in it.
(*) right/down increases, left/up decreases.
When using the color selector and the wand, if you click and drag(*) (instead of just clicking) this changes the threshold and shows you instantly what you have selected.
This has been around for a a while (at least since 2.8). Not really a secret since it is described in the doc, but who reads that

To experiment, create an image, fill it with Plasma (Filters>Render>Cloud>Plasma), star one of the two tools and click+drag in it.
(*) right/down increases, left/up decreases.