Kae'Yoss said:
Note that these are merely guidelines.
How exactly do you propose will this thing work? Put it on, fade from sight? What if you break the spell (by attacking)? Reactivate it? Stay invisible? Pull it of and put it on again?
Of course they're merely guidelines, or "suggested" rules for the creation of magical items. Note additionally that these items in the DMG are merely guidelines, or "suggested" items. You cannot assume that your GM or any particular campaign will have these items, or even have the spell "invisibility".
But I digress ... how do I propose the suggested continuous ring of invisibility works. (Well command word activated continuous item).
Personally I use it already as a continuous item. You put it on, you fade from view, as stated. Assuming it was 'activated' when it was last removed. There is also a command word (some continuous items do have command words, I view this as a continuous item with a command word). You can command it on and off as a standard action. If you break the spell the item turns off, and needs to be reactivated. This takes a command word and a standard action.
You're invisible from the time you become invisible, either by using the command word to 'turn on' the ring, or by putting on a ring already turned to 'on', until you break the enchantment... which you can do by taking off the ring, using the command word to turn it off, or by attacking (or performing other actions which would cause the spell invisibility to fade).
Edit: You can't command it on AND off as a standard action, you can command it on OR off as a standard action. Sorry.
^_^