Recalling during the race, the current pit stop traffic light system is primary automatic. Manual override if traffic is heavy, such as passing traffic to ensure no unsafe release.
When Shumi came in, the pit lane was clear, so I'd guess the system was on Automatic and once the sensors/check points on of four wheels, front and rear jack shows a green light (similar to the ones you see in McLaren), the main release will show a green light and release Schumi back in racing.
I'd guess during the first attempt of the wheel gun tripped the sensor when it dropped down, and sent a green light to the computer and thus released Schumi. I hope to hear more from Mercedes about the traffic light system.
This should be a tech feature from SkyF1 or BBC...