just a suggestion for using with ur powershot..
1.) mount on tripod
2.) frame subject with background
3.) set to manual mode
4.) set to ISO 400 (frame with 30% dark bg), 800 (frame with 60% dark bg)
5.) set to aperture f2.8
6.) set to shutter 1/4 - 1/2
7.) set flash power to 1/3 if subject is near (~1m), 2/3 if subject is further(~2m), full power (max ~3meters)
8.) fire off trigger and review picture to fine tune (ISO or shutter speed)
Give it a try..