The three modes of transistor operation are cutoff, saturation, and active.
The top circuit alternates between cutoff (when the switch is open) and saturation (when the switch is closed).
The bottom circuit is in the active mode when the potentiometer value is greater than about 21 k ohms. As the potentiometer value increases to the maximum of 100 k ohms, the transistor approaches cutoff. When the potentiometer value is less than 21 k ohms, the transistor is in saturation.
