In Agent Carter season 2 episode 10: Hollywood Ending, Jason Wilkes warns everyone that when a rift between the two dimensions closes, it sucks up everything within a twenty-foot radius into it. This is demonstrated when the rift begins to close near the end of the episode, and Sousa is almost sucked in. However, to hold himself to the ground, he grasps onto the handle of the device causing the rift (located directly below the rift itself, very much in a twenty-foot radius, and the device seems to be staying grounded just fine? Why is the device not being lifted into the rift at all? Why is it suddenly so heavy that it can support a full grown man being violently pulled into another dimension and not be floated up into the air with him?
From my understanding, since Sousa was able to use the lever on the machine, and everyone expected that to work, that the machine was not just a rift Creator, but also a rift maintainer.
This meant, at least for me, that the rift and the machine were never disconnected from each other. I assume that, if the machine was to be moved somehow, the rift would move with it, since they are connected.
This means that, even if the machine was sucked in, for every inch towards the rift (and towards the sky) it would move, the rift would also move one inch. You might think that this would just means the entire contraption would go flying out of reach instantly, but it actually doesn't.
It is vaguely explained during the season that zero-matter pull somehow works like gravity. The "other side" of the rift is so empty that it pulls things in, just like a plent is so heavy that it pulls things. Think of a negative distance, which is the same as a positive one, negative gravity is implied to act like gravity.
Now, when huge bodies have gravity, they also have a center of gravity, and the pull usually works like that. Even if technically a tree is part of the earth, you are pulled toward the center, not the edge.
Now if we look at the contraption, things are pulled toward the rift, not the machine, even if they are linked, because that's obviously where the huge amount of zero-matter is. it's the center of gravity of the contraption. If there was a way to separate the two, then the machine would go flying straight into the rift, as the force is implied to be very strong. But that connection is even stronger, because staying intact is necessary to avoid the rift simply closing. So as long as the connection stays intact, the center of gravity of the "pulled" contraption, including the machine, is in the rift, and the center of gravity of the "pulling" contraption, even if it doesn't include the machine, is still in the rift. Things don't move.
You can imagine it as you stading on top of a wooden pole. "Why isn't that person being pulled toward the earth, if everything else is?"
Yes, the person standing on top of a wooden polled is pulled towards the earth, but all the force of the pull simply pusheds the pole down, and as long as that wood is strong enough to not break because of your weight, you won't fall down.
In our situation, the wooden pole is the whatever-ium that is used to keep the rift and the machine linked together.