It's the prison registers, which date from 1790 to 1924. I don't know yet exactly which collection they have digitised, but if it's the one held by the National Archives, it contains details of name, age, crime, sentence, location of conviction, marital status, literacy level, trade or occupation and number of previous convictions.
This is pretty meaty stuff, and I, for one, can't wait to get stuck in. There's a particular story concerning one of my grandmother's brothers that I'd like to follow up. Family lore says he was put on a fast boat to America in the early 1920s to escape a second long spell of porridge, but I don't know what his earlier incarceration had been about.
I'll report here when I have more details about this new release. This may not be until the official launch at the Back To Our Past show.