Old televisions with glass picture tubes (seems strange to say this!) required a high voltage supply to operate. A transformer known as a flyback transformer steps up the AC (alternating current) used in the horizontal oscillator circuit to provide 20,000 to 50,000 VDC to the picture tube. The high voltage travels through a highly insulated cable with a wire clip on the end (known as the anode) and is attached to the side of the glass picture tube via a hole in the rear of the tube. A high-voltage capacitor is attached to or built into the flyback transformer.
If you look at the tv flyback schematic, the divider (for screen and focus) are tapped from the middle winding before the output diode. Due to this design, when a tv is switch off the high voltage from the picture tube (infact picture tube is a large capacitor) can't be discharge because of the output diode preventing the current from flowing to the divider. Thus the high voltage remains in the tube and can only be discharged with a screw driver connect between the cold ground and the flyback anode. You will hear arcing sound from the spark generated when discharging the high voltage. Best for non trained people to not muck around with these things.