This allows your 1966-1969 727 TorqueFlite to have partial kickdown from 3rd to 2nd gear without depressing the accelerator pedal to the floor.  It came on some 904 transmissions, but the supply has run out.  Chrysler changed the entire valve body in 1971 and incorporated a similiar design, but the later model will not fit the early valve body.  A&A recreated the design for the Pre-1970 TorqueFlites and have them available for you.  This bolts directly on the valve body.