Mike, take a closer look at the bottom of the bucket.
"Bailing tanks" for dewatering mines have clack valves in the bottom which open as the tank drops into the sump. When the tank is hoisted (heisted), the weight of the water in the tank holds the valve closed.
clack valve
inside the tank
It looks like the bucket in your pictures has a chain attached to the bottom. That chain is part of the dump mechanism.
The angled plate with the notch on the top edge, is hinged along its bottom edge (the plate is visible just above the bucket in your pictures). As the bucket is hoisted it tips the notched plate out of the way on its way up. Once the bucket is above the notched plate, it falls back into the pictured position.
The trick to the whole thing is the chain on the bottom of the bucket. If the bucket is stopped at the right spot, the chain on the bucket bottom catches in the notch on the plate. Now, when the bucket is lowered, the chain causes the bucket to tip over and spill whatever was in it, onto the chute.
There is a name for this setup..... I can't think of it right now. It's a pretty ingenious little arrangement.