Apparently it is also known as Snaked Whipping and I found a link to it, which shows the technique where you are finishing off a rope's end: http://www.survivalworld.com/knots/snaked_whipping.html
As I said, I use it to cover what one might term a sewn seizing. I stitch the two ropes together through their middle from one side to the other over a distance of about 4 or 5 cm - as long as you like to feel secure. Then ditto the top surface and then turn it over and ditto the other side. I then take a couple of stitches to hold it. These are also security for if the outer layer comes adrift. Then wrap the whipping twine tightly around the two ropes, covering all the stitching, as you would any standard whipping and take another stitch to anchor it; start your Spanish (Snaked) Whipping. The link shows what it looks like: you effectively take a half hitch with the needle and thread through (in my case) the top two strands of the whipping and pull tight. Then the same on the bottom two strands; back and forth until you complete the circle. Instead of tying two half hitches, I take another stitch and then send the needle through the heart of the rope for 4 or 5 cm on the long end, below the seizing, for extra security.
Thus far I've never had a problem with one 'creeping' or coming undone. I feel it's more secure than a standard racking seizing, but that probably says more about the quality of my seizings than their true efficacy.
A friend did some beautiful drawings of this type of seizing for me, for an article for Classic Boat. As is all too often the case, the article was never published and my work was never returned, otherwise I would have put the drawings up on the website.