That strange feeling that the plate is moving through molasses is caused by eddy currents. Eddy currents are electric currents that flow in circles every time you move a metal object in a magnetic field.
At the atomic level, eddy currents can be explained by a phenomenon known as the Lorentz force. When charged particles in the aluminum plate move through a magnetic field, they experience a sideways deflecting force—the Lorentz force—that causes them to move in circles in the plane of the plate. These circular currents create magnetic fields of their own, which interact with the external magnetic field to resist motion.
On the side of the plate with the cuts, however, the circular currents are largely blocked, and so less effective at generating magnetic fields. That’s why the plate can easily swing through the gap between the magnets.
The strength of the eddy currents generated depends on the speed of the charged particles moving through the magnetic field—so the faster the plate moves, the stronger the magnetic field produced, and the greater the braking force. You can feel this when you pull the plate through the gap by hand. Pulling the plate through slowly is easy, but the faster you pull, the greater the resistance. You can also see this by releasing the plate at different angles.
The strength of the force moving the charged particles is also dependent on the strength of the magnetic field. The closer the magnets are to each other, the stronger the field, so the effect is the largest when the gap is the smallest.
In the real world, eddy currents can be useful or a nuisance, depending on the circumstances. In the Drop Zone amusement park ride, which drops riders down from the top of a tall tower, eddy currents help slow the falling passenger cars when they reach the bottom. Metal parts in electric motors, however, sometimes have slots cut into them to prevent unwanted eddy currents from slowing them down.