In earlier times, the flashing frequency was generated using a hot wire element. This was slowly heated until, through its heated condition, it triggered a circuit breaker. Thus, it constantly went from one state to the other, unfortunately, not with the reliability demanded today. Since the introduction of electronics, which now advances so rapidly that the explanation of an indicator flasher soon became superfluous because this is now done, e.g., by the CAN-BUS.
Seen from an electrical point of view, the indicator flasher is an unstable multvibrator. It cannot maintain the once achieved state, but flips, after a certain time into another state. This is exactly what we require from an impulse generator.The actuall switching situations are achieved by two transistors. These are alternatively led by their respective bases, through a basic resistance. The (electrolyte) condenser is charged on one side by one of the remaining two resistances until the necessary tension is reached to trigger the transistor. This then switches not only the respective lamp on, but also provides the charging of the other (electrolyte) condenser and thus prepares itself for its own disconnection.The switch is not necessary for the indicator flasher in the vehicle. Just as little as a variable resistance which can be switched from above on the common plus-lead of both vertical resistances. Thereby the flashing frequency can be altered.
Modern indicator circuits - particularly those with an interval circuit - are constructed with integrated switching circuits and small controllers.