The cue hidden in the music

An original account of an invented dance rehearsal.

The dancers were meant to turn as the music softened. At first, we counted the beats aloud, and everyone turned together. When the counting stopped, half the group turned a beat early. The tune repeated a short phrase, and we had mistaken its first appearance for the later cue.

Our teacher asked us to listen for the lower note beneath the second phrase. Once we could hear it, we no longer needed the spoken count. The note was subtle, but it was reliable. I realised that a cue does not have to be loud to be useful. It has to be something you can distinguish from everything happening around it.

In this passage, explain the meaning of “subtle”.