There has been some controversy on the origin of yodeling. Some people say yodeling originally began in Switzerland, some people say Austria and then some people say Germany! The word 'yodel' comes from the German word jodeln, which gives us some insight into where the origin of yodeling actually comes from. The first 'yodellers' were around in the 1500's, and it was a form of communication for farmers in the Alps to bring in the cattle in the evening.
From the earliest 'yodellers', yodelling requires on knowing where your voice 'breaks' or in other words, where your voice goes from your chest voice, to your head voice. This then creates different pitches and sounds which goes over and under each other, both high and low. It takes awhile to get the right technique suited for your voice.
In group yodelling, solo singers often start off by singing quite low and relatively slow, then on top of this, the other people in the group start humming around the solo singer and is even sometimes improvised. This is known as 'natural yodel singing'. Songs that include 'natural yodel singing' are "Zäuerli'' and "Ruggusserli'' which are sang in the Muoto Valley along with another song called "Jüüzli'' which is sang with a bigger group of people.
In the video above, there is a group of people singing the Swiss song, Zäuerli.
sources: http://www.myswitzerland.com/en/yodeling-it-s-all-in-the-voice.html
http://www.wikihow.com/Yodel
Youtube.com
