This tiny gizmo is called an electronic dot sight in the manual and a Tactical 4 Reticle Sight on the box. Cost me about 35usd - sight + flash bracket mount.
First it needs to be calibrated (just a few screw adjustments) so that the center point hits exactly the same place as the camera's center point.
It has 7 intensity levels for the cross-hair (other shapes also settable) so can be used in any light condition and any lens. Recalibration only needed if changing camera bodies.
It has only one function - to accurately target a subject instead of using the viewfinder or live view when camera is tripod mounted. Focus will always be accurate even if you pan with the camera on the tripod as long as the subject is within the cross-hair. Shutter pumping probably required but not tested yet so not sure. The sight and shutter are not inter-connected in any way.
I saw some bird photographers using it on their tripod mounted 500mm and this device was universally recommended as a must-have. In addition they have a long range laser-pen to help the focus in lower light or very far off objects. Got one of those too - tried it in pitch dark and could see the beam go at least 3km but of course dot was too small to see.
Other people mount the camera on a shoulder mount and use the rifle sight + a remote shutter. Principle is the same though.
Pic is the rig of one of the guys - each and every shot he took without looking into the viewfinder and EACH was pin sharp.
Any thoughts? Suggestions?