[...every FBI agent carries one...]
Big deal.
The Z-series light was designed to be used with the so-called "Rogers technique", which allows one to utilize the ring and pinkie finger of the off hand to help stabilize the grip, while holding the light between the index and middle fingers.
This was thought to be an improvement over the "Harries method", in which the light is gripped in an overhand fist (in the off hand). The Harries is a wrist-over-wrist, using isometrics. The original SF configuration (6P, E-series) is perfect for this application.
Personally, I find the Rogers technique somewhat awkward; which was why I was recently in a position to trade away a slightly-used 6Z...
Most people who were trained on the Rogers, then shown the Harries, tend to prefer the Harries.
As a point of interest, Mike Harries was an Arizona deputy sherriff. Bill Rogers was an FBI agent.
You guys would no doubt have a fecal hemmorrhage if you knew just how much of your federal tax dollars are spent on toys that never see serious use...
Steve