Here is a mod which colorizes the attacker (green) and the target (red) of the selected player in a replay. It goes OOS, but you can click the warning away. Don't use it in games or as a spectator of live games though. The code is from elexis, I only packed it in a mod.
I viewed some games. I the fast sniping of sanafur and Stockfish is done by
-the firmware of the mouse
-or some software that translates a held key
While they hold this clicker-key, the repeated mouse clicks are automated in a high frequency. They can move the mouse meanwhile fast over units that shall be sniped. Depending on the speed of the cursor movement a target can be assigned a different number of attackers.
The targets are quite near of each other. The attacking units are random from the army (with big gaps).
The sniping is not so sophisticated, like choosing wounded units and assigning correct number of projectiles to take one out, as one would expect from a script.
The clicking is done in combination with held ctrl-alt key. Ctrl has the effect that the attacker will attack a unit when clicked on it, but when cursor goes over a spot over the ground the unit gets a attack-walk command and thus attacking the next target. This is not harmful like a walk command which would have been send without the ctrl button. Alt gives the command to only one unit per click and removes it from the selection.
When the attacked army is on a field, the attack-work does not work but rather the field gets attacked by a lot of units.
You can see in the following plots that sanafur and Stockfish gives a high number of attack-walk commands during sessions of sniping. Not in every game, but usually. So occasionally they can hover over good selectable units.
This is a form of cheating and abuse. E.g. in minecraft one can get a benefit by clicking fast, see:
https://cpstest.org/kohi-click-test.php
attack_viewer.zip