When to play sostenuto pedal

I know it's not used much but I just would like to know

The sostenuto pedal is the middle pedal on the piano. It is used to sustain certain notes, similarl to the damper or sustain pedal; however, the sostenuto pedal only sustains your selected notes while others can be pressed and released without being affected. This piano pedal is played with your left foot and can be played in conjunction with the damper pedal.

Step1
Place your left foot on the middle piano pedal. The ball of your foot should rest on middle pedal and your big toe should be in line with the pedal.
Step2
Keep your heel on the floor to enable you to press and release the sostenuto pedal when needed.
Step3
Play a note or chord and keep the keys pressed down.
Step4
Press the sostenuto pedal with your left foot.
Step5
Take your hands off the keyboard and the note or chord you played will be sustained.
Step6
Continue playing your piece of music, and only the specific note or chord you originally pressed will be sustained.
Step7
Release the sostenuto pedal to end the effect.

The damper and soft pedals can be used while the sostenuto pedal is depressed. The trick is to make sure that no pedals are depressed when trying to catch the notes to be sustained by the sostenuto pedal. Once these notes are sustained, the soft and damper pedals can be used without affecting your notes.
The damper pedal can be changed while the sostenuto pedal is down without affecting the notes held by the sostenuto pedal. This pedal is ordinarily used in organ-like textures to sustain long bass notes below changing harmonies ("pedal point").

The only trick is to not let the sostenuto pedal up until you really and truly want to release the sounds you have been sustaining.

To properly play the sostenuto pedal, the note or notes that you want to have the sostenuto pedal play must be played and held by the fingers until the sostenuto pedal is FULLY put down to the floor or depressed. The right pedal must not be depressed at the same moment the sostenuto pedal catches the notes to be held, because then ALL the dampers will be caught by the sostenuto pedal and all the notes will be held instead of the specific note or notes you want held. But once the sostenuto pedal has been put down, the pianist can go ahead and use the damper pedal as much as needed. The note or notes caught in the sostenuto pedal will continue to be held through any changes made by the right pedal. The sostenuto pedal has to be kept completely depressed when you are using it; even the smallest, tiniest, bit of a release will result in the catching of other unwanted tones.

It takes a bit of practice to get used to using it correctly, playing the notes (while making sure the damper pedal is not on) and then depressing the middle pedal quickly.

There aren't a lot of pieces in the standard piano repertory that explicitly call for the sostenuto pedal (the middle pedal), so I can't think of a "standard" way of notating it.

Nonetheless, I can think of two ways of calling for it where the notation would be clear and obvious:

1.) Create the pedal marking as you would for using the damper (sustain) pedal, but write sostenuto (or sost.) under the pedal marking. As an example:

______________|
sost.

2.) Notate the music in a way in which the only conceivable way to play the piece would be to use the sostenuto pedal- for example, write a left-hand whole note with a staccato part calling for both hands to play above it while the note is sustaining.

Here is a video on Youtube which uses this pedal:
https://www.youtube.com/watch?v=OmLbj7sKV_I

I have never seen a song where it is used