It is activated when the virtual piano is displayed.
Pressing a key generates a note. The note length depends on the pressing duration, as if the tempo was 60 beats/mn, i.e. pressing a key for 1 second gives a crotchet.
The lowest keys have special meanings:
Effective recording starts on the first MIDI event. At the end of recording, the MIDI events are translated to ABC and inserted in the current tune.
It also accepts events from the computer keyboard. Each keyboard row gives different octave: the upper row (digits) gives the higher pitches. On a row, each key is a half-tone distance from the next one. For example, with a US keyboard, 'A' gives the middle C, and 'K' is G, and the lowest notes ('Z' and 'X') generate a rest and a bar.
There may be problems while key repeat is enabled. On X window systems, it's safe to disable it with the command xset r off.
To remove the virtual keyboard, just press Escape.