Clonidine
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Clonidine is a centrally acting antihypertensive (to lower high blood pressure) agent, used mainly for this purpose in the past. It has found new uses, including treatment of some types of neuropathic pain, opioid detoxification, and, off-label, to counter the side effects of stimulant medication such as Methylphenidate.
Clonidine is a centrally-acting alpha-2 agonist. It selectively stimulates receptors in the brain that monitor catecholamine levels in the blood. These receptors close a feedback loop that begins with descending sympathetic nerves from the brain that control the production of catacholamines (epinephrine, also known as adrenaline, and norepinephrine) in the adrenal medulla. By fooling the brain into believing that catacholamine levels are higher than they really are, clonidine causes the brain to reduce its signals to the adrenal medulla, which in turn lowers catacholamine production and blood levels. The result is a lowered heart rate and blood pressure, with side effects of dry mouth and fatigue.
An analogy would be the lowering the temperature of a house by holding a lit match under the thermostat connected to the furnace.