Agreed! Best "optical slave" operation often happens when you use a fibre optic cable to "connect" the camera's flash to the strobe's sensor. The key rule to remember is that "optical slaves" can be triggered by anything, including some one else's camera flash. (I have a dive light that can trigger one of my strobes. Even worse, my divebuddy also has a dive light that can trigger one of my strobes...). Covering your strobe slave sensor by using a fibre optic cable means that only light entering the cable will trigger your strobe. Hopefully this will only be light from your camera's onboard flash.
Masking / blocking the onboard camera flash is yet another "art". The idea behind masking is to prevent the onboard flash from adding any light to your pic (since it may screw it up due to uneven lighting because the housing blocks one side of the subject) You are now using big powerful macho external strobe(s) to provide all light. Basic idea is to not let any light get out of the housing (YMMV, this is VERY hard to do!). Some housings also have internal "lens shrouds" (black rubber thingy) that wraps around the lens to prevent light from the internal flash bouncing off the inside of the housing, entering the lens and screwing things up. So masking tries to accomplish 2 things:
- prevent light from leaving the housing and making uneven light on your subject
- prevent light from bouncing around inside your housing and getting into the picture and messing it up
So I suggest that you do some dry land training with various configurations and suitable subjects (I am willing to Fedex several cats, at your expense) so that you build an good understanding of your rigs behaviour before you splash.
P.S. I fired off about 700 dry land exposures this afternoon as I fiddled with different camera / strobe setting combos for my new canon g16 before our belize trip at christmas. Can't seem to be able to locate any of the cats anymore, so shipment may be delayed until after feeding time.