A switch is a flexible rod, typically used for corporal punishment of the birching type, called switching after it, especially when using a single branch: multiple branches are rather called a rod, a less flexible single rod is rather called a cane, an inflexible one a stick; a paddle is broader but hard and flattened.
It makes an ominous 'swoosh' sound, rather like a whip, and can be agitated up and down quickly, so the lashes can rain down on the victim, who is usually a spankee, mostly bare bottom so it can 'bite' the skin. While young children usually suffer it over the knee (or rather the lap), it can be more painful if the discipliner makes more elbow-room by ordering the punishee to lie or bend over an object, which can, especially if standing, increase the humiliation by exposing the genitals.
Switches are most efficient (i.e. painful and durable) if made of a strong but flexible type of wood, such as hazel (also use for a very severe birching) or hickory (see hickory stick; as the use of their names for disciplinary implements, without specification, and as verbs for lashing, indicates, birch and willow branches are time-honoured favorites, but branches from most strong trees and large shrubs can be used, often simply nearby from a garden, an orchard or the wild.
Making a switch is simply called cutting, as it only involves cutting it from the stem and removing twigs or directly attached leaves as those would lessen its sting (hence deliberately left on for sauna use). For optimal flexibility it is cut fresh shortly before use, rather then keeping it for re-use over considerable time.
Parents in the United States (where the paddle is a more common spanking implement) are alleged by some non-Americans to threaten disobedient children with gifts of utilitarian coal and switches for Christmas should they not reform their behaviour. The actual practice is rare to the vanishing point, especially as most people live in urban areas where suitable wood is less easily at hand for the old-fashioned woodshed treatment than in past years, and most modern educators consider such severe physical discipline cruel. It is often banned by law as child abuse.