“Every beginning is cheerful; the threshold is the place of expectation,” Goethe once noted. At any given performance, audiences are typically good-natured and generous at the start, eager for their expectations to be met. Beginnings are easy. Then comes that certain moment in a piece when one feels the audience swing — whether into disappointment, delight, contemplation, or bewilderment. It happens in the bat of an eye. Derek Brueckner’s “Collaborative Studio” exploits and extends this moment at the threshold.