This is a new one to me. And really, it bewildered me to the point of absolute frustration the first time I encountered it.
Let's say you're on the defensive VS a very wiley Ragna, he's not only pressuring you, it's "smart pressure". He's not going to do anything that leaves him with an opening.
So he throws out 5B>2B (!) (And with most player's reflexes that I know, if you happen to be standing, 2B will most likely hit...so you instinctively go to block low).
Then he throws out a curve ball:
2B>6B (!) and while you were blocking low, expecting more lows to come out, he caught you with an Overhead which in Ragna's case, can lead to really nasty combos and Heat.
In a nutshell: 5B>2B (wait a tic) 2B>6B...
That said, the rule of thumb that's been taught to me is to always block low, and only adjust your guard habits to anticipate overheads...however this is not as simple as one may think. Just being aware of all the overheads and lows that pose a threat is a daunting task.
Jin's j.B is another problematic issue, as either I will try to use an anti-air to hit him out of the sky or it out-pokes me, or if I'm careless I get hit with it as an overhead. This goes for any jump attack, really, but some overheads come out really fast, or at least faster than I'm used to. It gets worse if all I'm stuck doing is guarding Jin's j.Bs, or if he does a normal guard string and IADs into j.D on crossup. I get caught by Guilty Gear's Dust Attacks and jump attacks with back hitboxes in a similar way too.
Learning what to look for in these mixup tricks and how to properly block has been said to be a major difference between how American and Japanese players play. So I think it's a good idea to beware of all the little dukes and tricks and pseudo-mixups that people have been caught with (if this thread could be used to list them?). Learning this stuff isn't matchup specific either, but at the very least I want to:
1.See right through the trick mixup (knowing what to expect).
2.Find a way out safely (safe movement/dodging/IBing at the right time).
3.Punish the trick (this might be more character specific though).
4.Possibly use some of these tactics for myself or be aware of the risks they pose.
Even if I don't use every character, I use quite a lot, so learning this stuff would help me, a lot.