Expression

Need to control parameters on you effect pedals? Turn the knobs.
Need to do it mid song with your feet? Get an expression pedal! (if your effect pedal permits). Expression pedals are used to control certain parameters on electronic music equipment such as digital amplifiers, rack effects, stomp boxes, MIDI controllers, and keyboards. The pedals do not actually create to the sound themselves, but remotely control certain aspects of the device they are connected to.