If you are looking at this you probably already own an Electra MPC Guitar and are looking to expand your collection of their vintage MPC effects modules.  The Filter Follower is #6 in Electra's series of 12 modules.  Their catalog describes it as "Synthesized self triggering envelope effect, artistically controlled by the manner in which string is attacked. A classic studio effect at your fingertips."  It's like a wah pedal effect that responds to your pick attack instead of rocking your foot back and forth.  Great Hendrix effect!  It has a switch on top for its "ENV" or "TRIG" setting.  Either one gives a different feel to how it reacts to your picking.  This is one of the better MPC modules and an excellent one to add to any collection.