Here is a very good explanation from the BBC
My cable company, Cox, is only one of three or four cable companies in the USA that offers the 24 BBC World News Channel. This channel is not seen in the UK. During the election coverage, BBC World News switched over to the BBC News in the UK so I was watching what was also been seen in the UK.
Some years ago, PBS made an agreement with BBC and went live with BBC coverage during an election. I did not get to bed until 1:30 a.m., as the coverage -- and that swingometer -- kept me glued to the couch!