Itching is a funny phenomena. It is the direct result of damage to cells in our skin. As a result of the damage, substances like histamine (the amine that causes widening of blood vessels and plays a role in allergic phenomena) are set free which, like any other free item. The signals of this irritation, skin to how pain signals are sent, are sent to the central nervous system.
One can compare the nerve fibres to telephone lines and the central nervous system to a switchboard. The switch-board routes incoming telephone calls and makes sure that only one like at a time is connected with each telephone number, Unlike the telephone lines, which sometimes get jumpled up, the nerve signals move on exclusive lines.
By occupying the line you can prevent others from having a conversation with the same number. Similarly, the itch message can be blocked by occupying the central nervous system with another signal, for example one generated by scratching. If another less irritating activity can substitute for itching, the problem goes away on its own. 