That’s from the Help file:
Quote:

Chord Step Advance

Use the Ins and Del keys on the numeric keypad to step advance on any track by one chord. The track MIDI data can display on the piano, guitar, lead sheet, drums, and notation window(s).

This feature advances the current track and displays the next group of notes on that track. For example, if the current track is set to the Melody track, pressing the chord advance buttons will display the next note or chord of the melody. The Chord Advance feature is a great way to study the notes being played, and to navigate around the track.

Unfortunately, the Big Piano Window does have a bug currently. While the Chord Step Advance buttons display chords nicely on the Guitar Window, the Big Piano Window does not display chords via its Chord Step Advance buttons. It used to be working with earlier versions of BiaB – thus it’s definitely a bug with the current version BiaB 2020(720).

Martin