These sheets of paper are quite literally one-sided. As in, one side doesn't exist, and is just an empty patch of air.
…Except things that pass through the "empty" patch of air don't come out the other side of the sheet, they just seem to vanish "into" it. The Foundation dropped a hamster into it, and now it's gone. Then, a D-Class reached through the sheet and picked up a beaker on the other side.
…Except his arm still didn't go "through" the sheet, and now the Foundation has two beakers, one of which didn't exist before. Plus, the D-Class says that his arm feels strange, and there's a weird smell coming from where he reached through. Perhaps the smell…
of a rotting arm, somehow severed and left to sit between dimensions?