Creates a seed for segmentation and then uses that seed for region growing.
The seed is generated by thresholding with the chosen intensity range, then keeping the island where you Ctrl-clicked.
You can certainly do this, by simply using the Paint effect and then using Grow from seeds effect. Local threshold just automates painting of the seed (replaced 5-10 clicks by one click).
When the island at the clicked position is determined, the thresholding result is eroded to prevent small, thin connections between islands leading to adding unrelated regions to the seed. If the region you clicked on was relatively small then this erosion removes it completely and nothing can be drawn there. You can reduce the Minimum diameter parameter to avoid this.