The force is due to eddy currents.
Work Step by Step
The conductor moving within the magnetic field will have eddy currents produced in it, as explained on page 972, due to electromagnetic induction. Lenz’s Law tells us that the currents experience a magnetic force that opposes the motion of the disk, causing a braking action. The faster the disk is pulled, the more rapidly the magnetic flux is changing, the larger the eddy currents, and the greater the braking action. An identical argument applies when the sheet is inserted between the poles, so a force is indeed required to insert the sheet.