Two wires connect the control device (which could be a thermostat, float switch, limit switch or other contact device) to the magnetic starter. When the contacts of the control device close, they complete the starter coil circuit, causing it to connect the motor to the power lines. When the control device contacts open, the starter is de-energized, and the motor stops.
Two wire control provides low voltage release, but not low voltage protection. Wired as illustrated, the starter will function automatically in response to the direction of the control device, without the attention of an operator.
The dotted portion shown in the diagram represents the holding circuit interlock furnished on the starter, but it is not used in 2-wire control.