It's a device hooked up to the pedal etiher as an acoustic trigger or a mechanical trigger, which replaces the actual hit sound with a sample. Its almost a must for any metal band that plays double kick at faster tempos simpliy because it allows for an equillized kick sound that has more click. Sometimes snares are tiggered as well for things with tonnes of blast beats where the drumer may not be able to hit the snare at a consistantly loud tempo.