- Upstream Keys are added on the program bus or any ME as part of that Bus’s output, coming on or off as part of that Bus’s transition
On most vision mixers the keys on the Programme bank (i.e. PGM/PST) (the bus is the source pre-selector rather than the bank) ARE the main downstream keys in the desk. (That's certainly the case on mainstream Sony desks. You can chose to transition them on the PGM/PST transition area - or use the separate DSK transition buttons - but both actions drive the same keyers - usually 4 of them)
Upstream keys are the ones pre-keyed on ME banks, with the ME banks then being cut to on the PGM/PST bus.
It gets confusing on desks that don't have a PGM/PST, and instead have just ME banks (though often there is a 'not obvious' way of cutting each ME bank to mixer output so you don't have to nominate a fixed ME bank to be the equivalent of PGM/PST - the old GVG200 was like that) and in some 'all ME mixers' you have additional keyers downstream of those on the ME banks.
However on a conventional ME+PGM/PST mixer the keys on the PGM/PST bank ARE the downstream keyers within the mixer, though of course you can then have external devices keying downstream of the mixer, so it's entirely possible to have DSKs themselves downstream of the mixer DSKs.
On modern mixers it gets even more complicated as you can have multiple versions of the same ME bank with different keys on them, with multiple PGM outputs from the desk cutting to different versions of the same ME with the different keys on them, so when you cut to ME1 - your two main mixer outputs can cut to two different versions of ME1 with different keys on them. This is how modern sports production works. MEs and PGM/PSTS now have multiple outputs that you can configure to do splits.
You are the host broadcaster for a sports game, but are also the domestic broadcaster. You want a host broadcast generic replay wipe to go out on the host feed, but you want a station branded replay wipe to go out on the domestic feed. You want the replay wipe to transition between EVS replays on ME1. For this you configure two frame memory clip replay/ramcorders with the two different wipes. You configure the ME bank to do the transition with two ME1 outputs (one with the host broadcaster wipe keyed over, the other with the domestic wipe keyed over).
You configure the PGM/PST (or bottom row nominated ME etc.) to then cut to the ME1 output with the host broadcaster wipe on one PGM output, and to cut to the ME1 output with the domestic broadcaster wipe on another PGM output.
When you are cutting the match - you cut to ME and trigger the wipe as if you were cutting just a single feed of the match, and on a second PGM output you see the other wipe happen entirely in sync.
This is NOT what BBC News do - as they do their splits downstream of the mixer - but it is 'business as usual' when cutting sport events these days.