To begin with, all the atoms are in a state where the first laser beam doesn't repel them, so they can pass through the beam with ease. However, once they cross this beam they are struck by a second laser beam, which alters their spin state and makes them repellent to the first laser. The result is that atoms are become trapped on the farside of the laser.
“Our one-way barrier acts to put all the atoms into a subsection of the original container. In essence this is the same as the original demon, since the point is that the demon can apparently reduce the entropy of the gas,” Steck told physicsworld.com.
Fascinating stuff, this is.