Shenzhen Bay. Lined up in the Chinese line. Entered with permit and was asked for a phone number (+852 was okay), and all my fingerprints were taken.
Turned left, went to the 2nd floor and did the e channel registration. They said I would be able to use the e channel to exit from tomorrow onwards, but I'd need to use the manual lane to enter one more time to collect all of my fingerprints ("because they only collected the thumbs" at the registration).
I went out to Shenzhen for a couple of hours and successfully used the e channel on the way out on the same day.
I'll go to Shenzhen again tomorrow I guess.