If you are reading this, you may think that I am referring here to controlling volumes in say a classical piece. When you are playing a 'classical' piece you may very well see these symbols in the sheet music you are playing from.
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The 'open' end should be played louder than the 'closed' end; so the first symbol indicates that the music should get LOUDER as you proceed to the right; and the second symbol means you should get QUIETER as you proceed to the right.
Now if you have an expression pedal, you can of course use it in this way. Here is a YouTube video by Peter Hayward explaining this very concept.
Using the Expression Pedal - Electronic Organ
But this is not what I am on about at all in this Topic.