To change from one state to another, and the thing that makes the change. Two positions, and nothing between them.
Probably Middle Dutch or Low German swijch (a flexible twig, a bough); the electrical sense of a device that changes between two states borrowed the whip-like snap of the older rod only in the nineteenth century.
The simplest choice a system can be given — one state or the other, with no room held open for a third. Making the choice and holding the position are the same single act.