Its been said before about the correct gas, panicked diver 'wants the reg you have in your mouth' etc. But something that one should also keep in mind is clipping off a backup (or Octo if one wants to call it that).
Emergencies usually happen when ONE small issue is compounded by others. Then very quickly things get out of hand and we have a major emergency on our hands. Clipping of can be a dangerous thing if for instance (and this is a mere example, we could come up with numerous if we want to):
Buddy is OOG, slight panic (even DIR divers-humans who need oxygen to live- panic initially when there is no gas to breath, but tend to control this quickly)
you donate primary
try to unclip secondary
Buddy looses bouyancy and trim control and starts to shoot up/down, pulls you with him
(you still dont have unclipped your backup)
now you are OOG (and initially panic, this effects your boutancy as well now)
etc, etc,
This may be slightly far fetched, but you get the idea.
The thing I think that is important in an OOG situation, is knowing that it will affect bouyancy/trim, is to both have gas as quickly as possible in order to assess the situation and make decisions. The extra load of not having gas to breath while fidling with a clip is not needed. Hence a backup reg hanging from your neck.
my 2 cents
Emergencies usually happen when ONE small issue is compounded by others. Then very quickly things get out of hand and we have a major emergency on our hands. Clipping of can be a dangerous thing if for instance (and this is a mere example, we could come up with numerous if we want to):
Buddy is OOG, slight panic (even DIR divers-humans who need oxygen to live- panic initially when there is no gas to breath, but tend to control this quickly)
you donate primary
try to unclip secondary
Buddy looses bouyancy and trim control and starts to shoot up/down, pulls you with him
(you still dont have unclipped your backup)
now you are OOG (and initially panic, this effects your boutancy as well now)
etc, etc,
This may be slightly far fetched, but you get the idea.
The thing I think that is important in an OOG situation, is knowing that it will affect bouyancy/trim, is to both have gas as quickly as possible in order to assess the situation and make decisions. The extra load of not having gas to breath while fidling with a clip is not needed. Hence a backup reg hanging from your neck.
my 2 cents