? You can use the following keys:

p	pause/continue	s	toggle autoscale
] or c	go to next measure	l	toggle line/measure cursor
[ or x	go to previous measure	h	toggle help
+/-	increase/decrease speed

In addition, when synchronization is enabled:

,	shorten the duration of the current measure	ctrl-,	shorten the initial offset (idle time before cursor starts moving).
.	lengthen the duration of the current measure	ctrl-.	lengthen the initial offset.
w or save button	save timings, abc code and other settings to a file (called a preload file). Also works with dropbox. You can load a preload file with the score file button. (see usage instructions for more info)

looping:

In loop mode the first two clicks in the score set the left and right edge of the looping range. (subsequent clicks adjust the closest range marker) synchronizing:

When your audio file is synthesized from score, no manual synchronization is necessary. Otherwise (e.g. live performance or video) you have to synchronize the media manually.
By clicking in a measure you synchronize the *first* beat of that measure to the audio. After clicking, playback jumps one measure backwards for auditing how well the entry of the measure became synchronized. You can disable this backward jump by unchecking the "jump" checkbox
Clicking in the first measure synchronizes the first beat (of the music) to (the first note) in the media file. This sets the, so called, offset of the media file. Do not forget to look at the measure duration and offset numbers in the top right hand corner of the display. You can also adjust these numbers (more precisely) using the keyboard (e.g. while the music is paused).