Direct from engine compressor:

A Load valve is switched on from the flightdeck, power for the switch is available once the APU has achieved 95% + 3 sec. This energises the switcher valve solenoid, which vents the lower chamber (B) of the control piston and pressurises the top chamber (A). The piston will move down and open the butterfly valve. The bleed air will flow and the EGT will rise, at a predetermined value the Load Thermostat will start to open which will reduce the pressure acting on the top of the piston. This will cause the piston to move up by spring pressure and thus back off the butterfly valve. If the EGT rise is excessive then it could close the valve. The valve will modulate under the control of EGT. The Load thermostat is set at a lower setting than the acceleration thermostat setting to prevent hunting of the system.

An electronically controlled APU uses the same principle, but the ECU controls a servo valve in the load control valve instead of the load thermostat, see figure.