Signal transduction
An activated receptor can start a sequence of protein interactions. Kinases transfer phosphate groups, usually from ATP, to targets; phosphatases remove them. Either modification may activate or inhibit a particular target, depending on that protein. Small intracellular molecules such as cAMP and calcium ions can relay signals. Amplification occurs when one active component activates many molecules at the next step. Turning off receptors and relays prevents a brief message from becoming a permanent command.
One activated enzyme generates many cAMP molecules, which activate protein kinases. Destroying cAMP can help terminate the response even after it has begun.
Phosphorylation is not a universal on switch. Read what the modification does to the specific target.