The optimization takes effect on pictures which are originally smaller than the image section intended in the preview. Therefore they have to be enlarged. By this enlargement the pictures are loosing sharpness. Through normalization and unsharp masking of the original picture a visible improvement is accomplished.
Both following screenshots are showing the same preview without and with optimization:
The optimization also functions for automatically generated preview-pictures which were engendered from videos with a 640×340 size. This is a feature request by diegummiluder.com.