Ok, after giving it some thought I think I get what they mean. I have always gone by the rule that as soon as I take a full agonist opioid, the waiting period applies, regardless of the last time I took bupe. So if I've taken sub in the morning but decide to do a pst in the evening, I'll be waiting 24 hours after the pst before I feel it's safe to take sub again. Same if I did a pst straight after taking bupe, it's another 24 hours before I'll take anymore bupe.
But, just thinking aloud, bupe causes precipitated withdrawals because it has a higher affinity than other opioids, so it immediately pushes any full agonists out of their receptors and then only partially replaces the agonist effect. Considering this, if you were to take heroin straight after bupe, it would have little or no effect, as we're all familiar with, as the receptors are already full with bupe, and the heroin can't get in. So it makes sense that if you were to take bupe again straight after that, the risk of precipitated withdrawals would be much lower - the bupe won't be removing any heroin and causing precipitated withdrawals, because the heroin never got in in the first place.
I suppose it does follow that for as long as any sub is in your system and is binding to some receptors, the degree of precipitated withdrawals would be reduced. Personally, I still wouldn't risk it though. Not to mention, I never switch back to a full agonist from bupe for less than 3 days anyway - there's no point, as it takes around this long for the bupe to clear and for them to work at full effectiveness again.
I usually get by fine on around 2-3mg a day, but I've always noticed on the first day or two being on bupe I need around 6-8mg to feel comfortable. After that first day or maybe two, though, I'm fine to settle down to 2-3mg. I think you might find that you do feel a lot more comfortable on 4mg or even less in a few days.