Blend by Brightness is used to control the visibility of pixels on the active layer (This Layer) and the lower layer (Underlying Layer) based on their brightness. Each scale has two sliders. At the leftmost slider a pixel has a brightness of 0 (black); at the extreme right a brightness of 255.
Those pixels whose brightness falls within the values between the two sliders will be displayed (for the upper layer) and hidden (for the lower layer). Sliders hide pixels without removing them.
If a slider is moved while holding down Ctrl, the slider will be divided into two parts, which can then be moved independently of one another. Divided sliders set the tonal range between visible and invisible pixels, so you can achieve a more subtle effect with a soft color change.
Actions on the active layer (current layer):
Active Layer: Hidden 0-150, Visible 150-255
Moving the left slider hides dark pixels with lower brightness
Active Layer: Visible 0-145, Transition 145-200, Hidden 200-255
Moving the right slider hides light pixels with higher brightness