How much light does CNTL flash "spill" into scene?

It's easy to see the otherwise negligable light now. But how does this relate to scenes you would really like to shoot in real (photographic) life?

(If I cough during a silent moment in a piece of music in a concert hall people will frown at me. If I cough during a speech hardly anyone will notice. If I do the same during a drum roll  or a forte passage in the music nobody will notice at all. )

- The risk is in reflective objects in the scene.

Actually I had hoped that the control flash would have ended before the actual shutter opening occurred.

Bart