It was a feature in their 'Support Program' which was kind of like a onsite patreon. I was in it and used to lock updates 1 episode ahead for people that pledged a dollar.
It went really well, readers in general loved it, and those that complained didn't have to wait very long for the update to get unlocked as only the LATEST episode was locked (1 day wait for my one comic, 1 week wait for my other). I made about 80 - 100$ a month with it, consistently.
Tapastic removed it though because the payment system was not compatible with the payment system they were setting up for their phone ap for buying premium episodes, which later eventually lead to the tipping feature for other users.