In a thermostat, the bimetallic coil is fastened firmly at one end. The other end is free to move. Its motion, as the temperature changes, can be used to control your furnace. For example, if the coil tightens as it cools, the free end can curl toward a metal contact that starts the furnace. As the air heats up and the coil loosens, the free end curls away from the contact and the furnace shuts off. When you adjust the thermostat you move the contact. When the contact is closer to the end of the coil, it will take a smaller drop in temperature to activate the furnace and your house will be warmer. Sliding the contact away will keep your house cooler.