The M-Audio SP-2 is a sustain pedal for use with electronic and MIDI keyboards. It's compatible with anything that has a 1/4" output for a sustain pedal, which is going to be pretty much every digital keyboard out there. It's got the traditional design for a sustain pedal however, as it's not the cheap press down button that you'll see a lot of out there. This is for players who want a more realistic feel for their sustain pedal, somewhere in the vein of that you'd get while playing a real piano. It can be used for applications beyond sustain as long as your keyboard has the option for sending to another kind of pedal, however, probably 9 times out of 10 people are going to be using the SP-2 for sustaining, and that's what I've used it for as well. I've used the SP-2 along with an M-Audio Axion 61 MIDI controller and I've found it to do the job adequately for inducing sustain into your sound. I love that it has a realistic feel like a real piano would, and while it doesn't feel as sturdy as a real one would, it's close enough to get the job done. I personally don't have this at home but I'm thinking about making an upgrade to it as I've only got a simple press down sustain pedal as of now. This is a lot better and more realistic as I've found out while using it at a home studio where I work with the producer who owns it. The M-Audio SP-2 isn't going to cost you too much at all and if you're after a realistic feel all around for your MIDI controller or other electronic keyboard, the SP-2 or something like it is definitely a good thing to pick up.