Hacking Manual

Pressing that @ screen button

When changing to the hack mode, the LilyPond text fragment that describes the current note or rest under edit appears in a text box. Also its duration, relative to the whole note, is shown.
You can change them freely, and it is your responsibility to keep the syntax correct.

External reference

You can recall the GNU LilyPond syntax from the official manuals, which are available using a clickable link. Note that the fragment you enter goes to the score, along with the notes. Take samples from the manual that fit in that syntactic context.

Group hacks

You may want to hack an opening mark of a group to one place in the song, and leave it incomplete temporarily until you reach the other end of that group. The group checkbox works like other groupings (e.g. slur, crescendo) to support this. That is, unticking the checkbox will submit both ends at once.

Duration

If you change the duration with the hack, it is important to update the duration value manually. Otherwise the whole song after the hack may shift in time when the hack is later overwritten. Time shifted scores may go off page and are really hard to recover.