Country Girl, the bar across the bottom is commonly called a slobber bar and, on this particular bit, is more for show. The original purpose of those bars was to prevent a rope from getting up between the shanks and doing damage to the horse's mouth during ranch work.
The angle of the shank dictates how much warning the horse gets before the bit is actually engaged. A bit with straight shanks gives very little warning so the pressure the horse feels goes from nothing to full power with very little time between.
The different port sizes change how the bit works on the mouth and where the horse feels the pressure. With a low or no port (mullen) the horse feels more pressure on his tongue. An average size port like the one posted would work almost solely on the bars. When you start getting into the higher ports like a Cathedral port, then it works on the bars and the palate of the horse.
Here is a thread that talks about all the different types of western and shanked bits if you want to learn more. Bit Information (Curb and Western type bits and hackamores)