You are talking about a loop pedal. Have a look at loopers delight for info on this.
http://www.loopersdelight.com
On the mpc you plug a foot switch which is nothing else than a simple switch adapted for foot use, if you know what I mean. All it does is repeat one of the functions on the front panel, if your hands are tied up when you want to punch in during MIDI recording, or you could play one of the pads with your foot for example.
If anyone is planning on buying one, make sure you get one with the right polarity, or one that has a polarity switch on it. They are some times called sustain pedals. If you are unsure, take your mpc with you to the store to see if it is compatible.
You could also make one yourself, it is really simple.