ECS is an artificial dance constructed by the Arthur Murray studio's to make swing dancing accessible to ballroom dancers.
It's straight six beat with no syncopations or anchors, so like most ballroom dances has little or no contact with the music.
In fact, as ECS is 6 beat it will always be 'off rhythm' because most music is phrased in 8 16 and 32 beat blocks.
A experienced Swing (WCS, Lindy) dancer (as opposed to a intl jive or ECS dancer) will lenghten, shorten, anchor and manipulate the elements of the dance so it always corresponds to the music.