Why Does the Middle Pedal Sustain Only Notes Already Held?

The middle pedal on a grand piano is often called the sostenuto, or selective sustain, pedal. It is not simply a weaker version of the right pedal. The right pedal raises all the dampers, while the middle pedal holds only those dampers that are already raised by the keys at the moment the pedal is pressed. Notes played afterwards are still damped when their keys are released.

For example, hold a low C, press the middle pedal, then release the C and play short chords above it. The bass can continue while the upper chords remain clear. If you press the pedal before playing the C, that note will not normally be selected. The skill is therefore not merely keeping the foot down. It is coordinating the order of hand and foot: first raise the target note's damper, then let the pedal catch it.

This is why the middle pedal can preserve a bass foundation or an inner voice without sustaining everything else. It does not automatically prevent muddiness; the player must still hear whether the held note conflicts with later harmony. Also check the instrument itself. On many upright pianos, the middle pedal is a practice mute that lowers felt between hammers and strings rather than providing true sostenuto.

https://usa.yamaha.com/products/contents/musical_instrument_guide/piano/selection/index.html
https://hub.yamaha.com/keyboards/k-how-to/how-to-use-keyboard-pedals/

If you use <a href="https://apps.apple.com/us/app/scoreflow/id6796205656">ScoreFlow</a&gt; to organise practice, make “play the bass first, then catch it with the middle pedal” one focused task. The test is whether the bass remains while the later chords are not prolonged with it.