This is the second part of M&G -- looking at electromagnetic induction.
When the copper coil swings past the pendulum, it experiencesachanging magnetic field, which induces an EMF into the coil, but as the electrons have no where to flow (it is not a closed circuit) there is no current.
By applying a copper rod between the ends of the coil, a current will flow around in a loop. When induced current can flow, it provides a resistive force, opposing the motion of the coil relative to the magnetic field.
So, by putting the rod between the ends of the coil, this allows current to flow, which produces a resistive force which will slow the pendulum more quickly.