As to how they worked: it's described at 6:41 in the video. There's a little notched cylinder in the bottom of the small powder box behind the face of the frizzen. Lowering the frizzen engages a pawl near the pan , which pushes a ratcheted wheel to turn the cylinder to drop a small amount of powder into the pan, and turns it further to seal the powder box.

As to whether it could work, it could well work just fine: the notched cylinder could be a very tight fit, and well-greased to seal the powder box. The hot gases from the powder flashing in the pan would not be very high pressure when they reached the sealed powder box- it's been flipped forward by the cock- so perhaps there was never a small explosion there. If the powder was a little damp and clumped, it's possible that little cylinder didn't do a good job of metering it, though, and it seems a little hard for the shooter to quickly verify if priming actually happened- opening the frizzen seems as though it would automatically prime the pan again, when closed. It is also possible that anyone who bought one of these simply couldn't bring himself to shoot it, in the short time before they became obsolete, because it was just too complex and he didn't want to break it by accident, and that's why surviving examples are generally in very good shape. But unless somebody comes by here with a diary entry about one misfiring, we are unlikely to ever know. Antique guns in this fine condition condition can be lose significant value if shot, as it can damage or degrade them, so the current owner is unlikely to want to test for reliability.