The sharding system is operating all the time. Otherwise you would very clearly see when it doesn’t… Simply by your screen being filled with way too many players or you not even able to stay logged in due to the massive data flow.
The cause is simple. The way the executive is set up in the code. It doesn’t look exactly like what I write below (since I have not much experience with code syntax), but the functions principle follows basically this routine:
If [shard] = [player count] < [player minimum] → transfer all remaining players to a different shard and close the current one
If [shard] = [player count] > [player maximum] → transfer currently logging-in players to a different shard and balance out the over-crowded one by blocking join access for some time
The moments we see active sharding happening is when either a GM or Server Admin is executing it manually or when one of those 2 functions is executed. And of course if you join/leave a group, since groups are always prioritized in sharding.
Hence why I say that players should have manual control over it when playing solo. So we can select a shard we want to use and play there, in worst case click again and phase over to the people you just talked to in case of RP. In the worst case the game would tell you in the list that shard [xy number] is full and you can’t transfer over. But in such a case even the automation wouldn’t help, since the function of the system would shard you away anyways in such an extreme case.
Extra effort we pay for actually with our subs. That sub is not just paid for Blizzard to earn money but also to upkeep the services which includes the servers too.
As I said above in the main post. 4 PM MEZ on a Monday is the time when usually many players on AD are playing. And I know that because I changed the zone, went back 20 minutes later and the Morqut Village was crowded with players again and all rares that previously were visible on my map were gone. Simply because I was on a different shard now. And it took 20 minutes of going to another zone, do something there and go back.
Oh and just as a sidenote, my main game Destiny 2 uses also sharding (like so many other online games with open worlds). Usually public areas for gameplay are seperated in their design like this:
Zone A ↔ Intersection ↔ Zone B ↔ Intersection ↔ Zone C
In Destiny 2 I first have to go from Zone A through the Intersection to Zone B and kill some enemies there or do anything, when I go back through the Intersection to Zone A, I get a different Shard of Zone A.
It’s a very simple system actually. WoW’s is afaik not that much different when it comes to traveling.
Again, hence why we, the players should be able to select our current shard manually too, alongside the automation so we can fix the flaws of the auto system on our own with 2 clicks on the UI.
Hence why I am making this post, so you and others see our problem and realize that there should be a fix for it.
I was just writing the last section, what happened? 
