1) The rules are exactly the same as for an analogue (disc) tacho.
2) Presumably you have never been taught these basic rules, so get your employer to send you on a course. That way, when you know the rules, you can use them to you advantage. If you don't know the driving period rules, do you know the rest rules? A course will also count towards your 35 hours training for your Driver CPC even though these do not come into force until September this year.
3) Beware with digital tachos. If you are in a queue or something where you are just edging along, the digital tacho will 'assume' you have driven for 2 minutes for each little nudge forwards. Recently a lot of drivers have found the digitacho reckons they have done alot more driving than they really have.