Let's see how we can get a nice picture on a flat (instead of a sphere-shaped) screen. The electrons are deflected too much to the left and to the right. This can be solved by replacing the voltage source by a capacitor. When the transistor is on, the current will now be drawn from the capacitor Cs. The voltage across it will decrease. This will cause the current to rise less rapidly (less than 150000A/s). When the transistor is off and the coil current is negative, the voltage across Cs will rise again. And that is exactly what we want. The coil current is now somewhat S-shaped. This correction is therefore referred to as the S-correction. We only need to take care that the average voltage across Cs remains 150V. To assure that, we cannot just connect a voltage source across Cs. The capacitor is therefore charged via the deflection coil. Choke Lp prevents shorting the voltage source when the HOT is on.