Click the card
Moves to the next card. The whole card is the button, so this works on a phone held in one hand.
Spacebar
Next card, without looking away from the screen. This is the one to use when you are running the scene out loud.
Backspace
Back one card, for when a line gets away from you and you want to take the run at it again.
Escape
Ends the run and puts you back on the full deck, where the settings are.
Two settings worth knowing
Hide your lines. Your own cards go blank and just say "your line". Run the scene, say it from memory, and press space to check yourself against the next card.
Set timing. Turn this on before a run and Go Offbook records how long you spent on each card. Play that run back afterward and the cards turn themselves at your pace, hands free.