From the top side, I know this is part of why knife play (without cutting) appeals to Me. I like playing with that kind of fear. Even though the bottom knows that I wouldn't intentionally break their hard limit, there's still that part in them that is afraid because of what I could do. Blade in My hand while they are bound and helpless, with no immediate escape unless I set them free. It's not anything that I even have to vocalize. I know the what if's that are running through their mind. What if I might break their limit? What if they are really in danger? What if I'm not as trustworthy as they led themselves to believe I am?
I love this type of play because, IMO, it's the most real. There's not a lot of risk involved in swinging my riding crop or a flogger. Even if I aim badly and wrap, or strike a little too close to the kidneys or spine, the fact that I have tendonitis and can't strike too hard makes the chances of doing serious damage relatively low.
On the other hand, my daggers are extremely sharp and have been known to "bite" the unwary. Using them requires a greater degree of caution than one might think in order to avoid accidental nicks. So the fear, in this case, is very real because the sub has to trust in both my integrity and my skill. No matter how careful I am, there's still a chance -- albeit a small one -- that she'll move the wrong way or at the wrong time and I won't be able to move the blade in time to keep from cutting her or that my hand will slip and he'll get more than a scratch.
"There's something that doesn't make sense. Let's go and poke it with a stick."— The Doctor