We have this type of hot plate in our house. It has a raised centre section that depresses when a pot is placed on it. The temperature control has 10 graduations compared to 6 for the other 3 hot plates, which do not have the centre section.    We just tried use it to boil some oranges to make orange cake. The water started bubbling over so we turned it down to 3 but it still kept supplying a high heat. What is the centre part and how does this hot plate work?  Edit:  The other three plates do not have this centre section. 
I managed to locate the cooktop instructions. The hotplate is one with a thermostat control and the raised section is the temperature sensor.  From the instructions:     The right rear hotplate control is a thermostat.   The control may be set anywhere between low and high but must be turned anti-clockwise back to off.      This control switches the hotplate on and off to maintain the set temperature.  