This may be easier to show rather than tell, but I'm on my phone here so forgive me for using screenshots of someone's YouTube video of their MSQ run, which I've marked up.
When folks say "look for the raised hand", it's worth noting that many of us play with the camera zoomed out as far as possible; it makes seeing the arena (and things that help make mechanics more readable) much easier. No mod involved.
So, as an example, here's a shot from that YouTube video. You can see the person is playing with their camera fairly far back, and I've marked up what folks mean by the raised hand:
The fist in the air is going to come down and splat that poison bubble, covering that side of the room. One thing you'll notice is that the puddles are scattered a bit randomly, but they stop
roughly around where the elbow of the arm that isn't raised is resting on the ground:
So when he drops the poison blobs, imagine a sort of line down the center of the room; stand on that line. As soon as one hand goes up, move to the opposite side; as long as you get out far enough to be past that elbow line you should generally be safe.
Hopefully that helps a bit?