Gravity bong = smoking device that uses gravity. This can be a bucket-bong, a parachute, a waterfall...probably more that even I haven't heard of. Instead of YOU sucking to fill the chamber with sweet, sweet smoke, gravity is used to create suction.

With a bucket bong, the force is created while you lift the (jug, bottle, barrel, whatever you're using) away from the water. Since there's a hole for air to be pulled in, air WILL be pulled in as the water level drops. Since there's some weed and hopefully some fire in the way, the smoke will be pulled in instead of air.

Now if all goes well, you should be holding a nice, big hit in your hand...so why aren't you smoking it?