A little help understanding DNA supercoiling
After studying this topic a bit more I came to the conclusion that supercoiling itself does not overwound/underwound the DNA any further that it already is. Supercoiling just relieves the stress introduced after DNA gets further twisted in the first place, it relaxes the DNA by decreasing the twist back to normal (10.4bp per turn just like normal relaxed B-DNA) (in case of positive sueprcoiling and vice versa for negative) and the extra tension that made the DNA twisted in the first place now increases/decreases the writhe for positive and negative supercoiling respectively. The authors might have considered a DNA molecule with higher writhe value also a more wounded form of DNA that's why they used the term 'overwound'

Please correct me if I am wrong