This is a Harken 009 ratchet block mounted onto the jib car/slide.  The 009 has a manual switch to turn the ratchet on and off but you can substitute the Harken carbo auto-ratchet if you would prefer (link is below).  By putting the ratchet block on the jib car/slide the need for a mounted ratchet block on the seat or on the side deck is eliminated.  This rigging configuration is popular among camps or daysailors who prefer to keep the seats and side decks free from hardware as much as possible.  For racing, it is preferred to have the block on the car/slide simply be a fairlead block and have the ratchet block mounted.  The most popular configuration is to mount the ratchet block on the seats.  This rigging configuration is linked below.