Hi Pelly. When first reading this and seeing the photos, you are drawn to the natural conclusion that the string will go through the loops. However, this would make removal, or slackening, of the string very time consuming, so the string is passed under the loops, thus greatly speeding up access for weeding, harvesting and the like. It probably helps “nip” the covers down a bit tighter too.
I agree, a great book, and my personal favourite from Charles writings. There’s a strange but enticing romanticism in gardening during winter, even if it’s a bit parky…