"...other sources and fencing masters, particularly later ones, do mention quite a few other secondary guards for longsword. There are some variations and discrepancies between authors of course, as well as different interpretations among contemporary researchers.
Many, if not most of these are considered only transitional guards, so just particular positions while in motion from one to another primary guard or end point of a strike, cut or thrust."
Enjoyed reading this article: http://mindhost.tumblr.com/post/151381536147/secondary-german-longsword-guards
I would disagree with the article in that I think the idea that "secondary" guards are transitional, i.e. point of movement, as this strikes me as received wisdom that has not seriously been thought about by most people.
When you think about it Day, Fool, Plough and Oxs are all highly transitional. Why is this? Think about it: they are absolutely rubbish positions to hold. If you stand in these positions and do nothing you will get splatted. Day and Fool go without saying but likewise with Plough and Ochs: holding a properly withdrawn Plough leaves your Weak out there, ready to be captured and blasted through while Ochs, while deceptively protective, leaves the hands extremely vulnerable as endless posts about "how come I keep getting hit on the hands while in Ochs?" testify to. You need to move through these positions to be protected, they are positions that temporarily block lines/invite/deceive your opponent as you pass through them on your way to doing something else. Ochs is useful as it threatens a thrust and discourages an attack to your upper openings, it only actually defends you though if you move into another position such as completing this thrust and entering a Hanging Guard. Likewise with Plough, it threatens a thrust but you are only defended when you enter Speaking Window or Long Point.
Contrary to this many of the "Secondary" guards you can't help but think that some are actually relatively good positions to just sit in and receive an attack, unlike the "Primary" guards. Which is the opposite of received wisdom: Hanging guard is wonderful to camp out in, likewise with Speaking Window and Barrier Guard.