IT WAS TAUGHT IN A CLASS I SAT IN ON BECAUSE oops caps,,, because you don't know where the back up is on all divers or if you can get it released from its holder or other things in top of hte hose. They are told by many they were taught the only source of air available is in the mouth of your buddy and inteh dark you can not see to find the backup or that the diver is prepared to give up the one they are breathing on. I have also had people ask me for air with their hand on my primary. I think this is a 50/50 thing strongly dependant on how they were taught. Many I talk to about this do not trust the back up to not being tangled and unusable. Unlike tech gear checkouts once in the water most all I know do not do that. I do run the drill with those I dive with to practice and underwater checkout for leaks and accessability to the buddies air. Then again most I dive with are diving to practice those specific things. Im going out thursday to do just that for another diver that just wants to experience such issues and how to work around problems.