It's a pretty standard approach for technical open circuit diving, and for recreational divers who follow that time tested procedure. There are a million threads here about that but basically:
The idea is that the donating diver is giving the victim what they know to be a working regulator. This type of gas sharing is built around the assumption that the OOA diver isn't going to just come up and mug the donating diver - who is unaware of what is going on - for gas. The donating diver is controlling the situation. Although from what I understand, a really panicked diver will go for the reg that they see in someone's mouth in any case, so you might as well be prepared for that.
The basic assumption is that the OOA diver has been OOA and need gas immediately. So as the donating diver, you immediately plug what you 100% know is a working reg into that oxygen starved, panicking diver. That significantly improves the situation, for whatever happens next.
At this point, the donating diver can take their time deploying the alternate for themselves. The donating diver has been breathing normally and has a minute or two before they absolutely need gas again. If there is any problem with the alternate, which is not uncommon, then you can move to buddy breathing or whatever, but you have at least ratcheted down the panic.
AND, of course, the only excuse for running OOA is a catastrophic gear failure like a burst LP hose. So try to make it so this never happens!
The idea is that the donating diver is giving the victim what they know to be a working regulator. This type of gas sharing is built around the assumption that the OOA diver isn't going to just come up and mug the donating diver - who is unaware of what is going on - for gas. The donating diver is controlling the situation. Although from what I understand, a really panicked diver will go for the reg that they see in someone's mouth in any case, so you might as well be prepared for that.
The basic assumption is that the OOA diver has been OOA and need gas immediately. So as the donating diver, you immediately plug what you 100% know is a working reg into that oxygen starved, panicking diver. That significantly improves the situation, for whatever happens next.
At this point, the donating diver can take their time deploying the alternate for themselves. The donating diver has been breathing normally and has a minute or two before they absolutely need gas again. If there is any problem with the alternate, which is not uncommon, then you can move to buddy breathing or whatever, but you have at least ratcheted down the panic.
AND, of course, the only excuse for running OOA is a catastrophic gear failure like a burst LP hose. So try to make it so this never happens!