Here's your
panicked diver "flailing around": at this worst case scenario point (after rejecting her regulator & ripping off her mask), it doesn't matter what kind of "safe secondary" air source regulator system you utilize to donate -->the panicked diver OOG whether actual or perceived, in her present cognitive state of mind & action, is heading immediately for the surface. . . You do your best as an assisting diver (as the Instructor in the video is trying to do) to recover some semblance of control on the way to the surface.
In thirteen years now consistently diving the 7' long hose Primary DIR configuration, I've donated for real in three non-panic circumstances --twice for for divers in Open Water low on breathing gas, and once for an Indonesian Dive Guide who had O-ring blowout on his yoke valve on initial descent (donated my 7' long hose, switched to my necklaced bungee'd back-up, shut down his tank valve, displayed my SPG showing 150 bar indicating plenty of breathing gas for us both --and we both elected to continue the dive, sharing gas for nearly 30 minutes dive time on a nice shallow 9 meter part of of the reef system at a dive site in Raja Ampat).