The single knot uses one corner of the furoshiki. You can gather the corner up in one hand to make the single knot.
But if you are using a large furoshiki to make a bag, a neater effect is achieved by folding the corner in on itself, once or twice, as if making a paper aeroplane. (See diagram: Fold each side of the corner to the middle, and repeat). This creates a neat strap of fabric to create your furoshiki style.