This technique has been embraced by Gunsite and many other shooting schools. It’s likely the most often taught technique at law enforcement academies. That is partly because it works just as well with flashlights that have the activation button on the end or on the flashlight body, and partly because it’s easy to use.
With the Harries technique you lock your wrists or the backsides of your hands together. To employ the technique, slip your non-shooting hand—which is holding the flashlight—under your shooting arm and then lower the elbow of your non-shooting hand by rotating your non-shooting arm at the shoulder. This applies pressure against the backside of your shooting hand and makes for a relatively stable shooting platform.
If this technique has a downside, it’s in assuming the position. It takes some time and often a shooter will start positioning his or her non-shooting hand across the body before the handgun is oriented toward the threat. This results in the handgun being pointed at the non-shooting hand or arm as the gun is being presented to the target. Be aware!
The best way I’ve found to employ the Harries technique is to first assume the neck index position with the flashlight illuminating the threat. Then, after the handgun has been pointed toward the threat, slip the non-shooting hand with the flashlight under the shooting arm and lock the wrists.