
I have spent decades of my life fascinated by the fact that Guinan falls into this stance, and Q keeps his distance. What in the hell was she going to do? Why couldn't we see it?
What about this theory? Guinan's bluffing. She convinced Q that her race has a power that is a blindspot for the Q continuum (perhaps some technobabble like the power to manipulate tachyons or something). We've already seen she has the power to sense other dimensions in Yesterday's Enterprise, so it's somewhat plausible. Perhaps Q came close to testing her once before. and this pose is how chose to intimidating him. Q's not sure if he believes her or not, but he doesn't want to risk being wrong.
In Simpsons terms, Guinan's using Bart's Touch of Death.
