The expression pedal on the Hammond organ (and its digital clones) is the foot pedal on the right side which controls the volume of the organ. It acts as a tone control as well- as you swell from soft to loud, the tone of the organ changes from mellow and hollow sounding to full and assertive. Since the organ is not a touch sensitive instrument, the expression pedal is the main way to add dynamics and life to your music.
Each player’s use of the expression pedal is unique. Some players use it a lot, and I see other players take their foot off of it completely and just use it as a volume control when a song gets louder. My introduction to expression pedal technique came a few years ago from Ondrej Pivec. He said “When you play a chord, it is like showing a special secret. Then you want to quickly hide it. Then, you can slowly bring it back.” You can hear Jimmy Smith doing this a lot; check out “Christmas Cookin'” for some kickin’ expression pedal stuff.
Another thing I learned about expression pedal was from organist Nate Shaw, who studied a little with Jack McDuff. He said that the purpose of the pedal was to bring out whatever you want to highlight in your playing at any moment. For example, if you stop playing with your right hand for a few measures and just play a bass line, you can ‘floor’ the pedal to really bring out the bass line.
Also, you can pump the pedal slightly during a bass line in order to give the notes a little bit of decay and make them sound like an acoustic bass. Beware though- pumping the pedal too much, as if you were tapping your foot on each beat, makes the music sound erratic and chopped up. It’s a common beginner’s mistake.