1. Zoom. This controls the spread of light from the flash. The numbers correspond to lens focal length, so 24mm is a wide beam spread, and 105mm is a tight spread. To see this in action, stand a few feet from a wall and shoot the flash at 1/64 power using different zoom settings. See the difference in how wide the flash pattern is.
So should I try and match the focal length of my zoom?
2. Power. Manual is manual -- the flash *always* fires at the same power output every time. You have to set the appropriate aperture (and shutter speed) manually on the camera. You set the power level from full (1/1) down to 1/128 in 1/3's of a stop.
This is useful when you are using the flash off camera, possibly on a light stand with an umbrella. You set the appropriate flash power and manual camera settings, and the flash puts out the same amount of light every time.
So does it act like FEC in Auto Mode(Flash Unit) in that you can reduce light exposure output i.e if you are trying to create a lighting ratio or a fill flash stop difference?
When using a diffuser (1-1/2 stop difference) which one of these would need to be adjusted to compensate