Jonny Greenwood on BBC

The National Youth Orchestra of Wales rehearse one of Jonny Greenwood's pieces for the Proms.

Instead of beating time, the conductor sweeps an arm across the orchestra, and each musician begins their note as the gesture passes over them, so the sound keeps shifting and never lands as one attack. One player says she has to almost ignore everyone around her and follow only that, which she has not had to do before.

A young member of the orchestra says it is an honour to play one of his pieces, and fascinating to hear what somebody she knows through guitars and electronics does when handed an orchestra.