The Harries technique is one of the more popular techniques out there, probably the single most popular technique among law enforcement.
The Harries technique involves holding the flashlight in your weak hand, crossing your weak hand under your gun hand, and then pressing the back of your weak hand against the back of your strong hand.
In this position you push your strong hand against your weak hand while pulling the weak hand against the strong hand utilizing the stabilizing power of isometric tension.
(Similar to how the Weaver stance works)
The Harries technique is more stable than the FBI or neck index techniques because it gives some extra support to the shooting hand.
Personally I find that my accuracy is much improved using the Harries over the FBI or neck index methods.
When used properly the Harries also makes it very easy to index the light and the sights in the same place.
The positioning of the hands tends to make it fairly easy to get the indexes aligned right off the bat.
The Harries technique is fatiguing.
While the isometric tension used in the technique helps to stabilize your shooting platform, it also requires muscle tension, and that gets old real quick.
The positioning of the hands is also pretty un-natural and it takes a good deal of muscle energy to simply hold the Harries position even if you aren’t using the isometric tension.
As a result you’ll watch guys on the line start to drop the light and the weapon down to about a low-ready position and try to bring the light and the weapon up.
The Harries is more of a committed position than the FBI or neck index.
It requires slightly more time and effort to get out of so you can transition into the FBI or neck index techniques.