Click the control or press S to activate.
The Spike Sharpness control, part of the Main Diffraction Spikes group, allows you to make the Primary and Secondary Spikes more fuzzy or crisp.
Higher Spike Sharpness values make the spikes appear sharper and narrower.
Tip: An image will often look most natural if the spike sharpness is matched to the size of the highlights, though in some cases sharper spikes can give an image a feeling of higher resolution.