Using as 1a(NO) contact form
When voltage is applied to the electromagnetic coil, magnetism flows as shown by the red arrow, turning the contact ON.
Using as 1b(NC) contact form
When voltage is applied to the electromagnetic coil, magnetism flows as shown by the red arrow and cancels out the magnetism of the blue arrow, turning the contact OFF.
*If the voltage applied to the coil is excessively increased, the contact is turning ON again.