I'm finalizing my translation (finally!).
"Moves the playback start point (which shows as a green bar) earlier in time
The red and green bars do not get drawn until you have started play, or at least created the time base."
What is the time base here?
Also, I see that in version 0.9.2 (debian package) the red and green bars appeared immediately as soon as I hit play, while in git version nothing appears at the edges, I see only a yellow mustard bar moving from note to note.