Rennie went into action, so swiftly that for a startled moment Drew was left gaping at empty space.
For a moment, Rennie was left gaping at empty space, since he was so startled by Drew.
mnli hypothesis: For a moment, Rennie was left gaping at empty space, since he was so startled by Drew. premise: Rennie went into action, so swiftly that for a startled moment Drew was left gaping at empty space.