Ten members of an improvisation ensemble called soft rocks leave it all to chance as they map out choreography on the fly in Stories of Spirit and Spirits at Main Street Landing Performing Arts Center in Burlington. The ever-evolving experiment in sound and dance — directed by local mover and shaker Jessie Owens, with music by Vermont Symphony Orchestra project conductor Matt LaRocca — taps into the ethereal nature of performing arts. The spin of a bottle or the pull of a card trigger in-the-moment responses from the artists, setting in motion alternate outcomes for each showing. One thing is for certain: Expect the unexpected.