Adjusts the angle of a spotlight's falloff.
This button replaces the Region Zoom button when a spotlight viewport is active.
Click this button, then move the mouse in the spotlight viewport to make the cone of the falloff narrower or wider (the falloff cone is shown in gray, the hotspot is in blue).
Hold down the CTRL key while moving the mouse to lock the initial angle separation of the hotspot and falloff cones.
You can’t adjust the hotspot larger than the falloff, because that would change the falloff value. Likewise, when you reduce the falloff, it stops at the hotspot size (in both cases, separated by the Angle Separation, specified in the Preferences/Rendering page).
To override the separation of the hotspot and falloff parameters and cause the parameters to affect each other, hold down the SHIFT key.
The inner hotspot appears to shrink as the difference between the hotspot angle and the falloff angle increases.