That point is formulated in a pretty difficult way, even though it’s extremely powerful once you learn to apply it.
The key part is that if you manage to make yourself look vulnerable, you can bait your opponent into certain actions, which you again can counter. And one way to do this is to use blockstrings that end in moves that leave you at negative frames (and thus look vulnerable), but leaves you at a spacing where your opponent will likely attempt to counter-attack (again, because you LOOK vulnerable while you’re actually safe). You can also do this with single moves.
Let’s take a look at the video example provided in the handbook and see what happened:
After hitting Choi with a custom combo, Valle does a cross-up j.LK, followed by c.MP xx high tiger shot. This leaves him at negative frames at a pretty close spacing. Because he looks like he did something unsafe, Choi activates his custom combo in an attempt to punish, but in reality Valle was safe. This allowed him to punish the CC-activation by uppercutting.
There’s more to it than that, but for the reasons discussed in the handbook, that’s why that video example was provided. Valle used a blockstring to make himself look unsafe, and because Choi reacted in a predictable manner, Valle was able to punish him for it.
If you want to learn more about how to apply this (and have it explained by someone far better than me), I recommend reading through this discussion page ( illitirit’s posts in particular, obviously). The examples provided are great for understanding and utilizing this topic.