Lets define the basics and go from there, it will make more sense. The two main vessels we are talking about is the descending portion of the aorta and the subclavian artery, which branches off it. Coarctation of the aorta, is another word for narrowing, and postductal means this narrowing occurs below the left subclavian artery, or distal to it.
Since the subclavian branches off before the coarctation occurs, it's able to receive it's normal supply of blood. So you will have nice, strong upper extremity pulses.
The blood supply to your lower limbs is reduced, due to the narrowing, and as a result you will have weak, lower extremities.
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For a coarctation that is proximal to the subclavian, the right subclavian will have it's normal blood supply, so the pulse will be strong, but the left subclavian will not receive it's normal blood supply, hence the weak pulses.
This is all associated with Turner's syndrome, and they will present with rib notching in the distal example, because the blood back flows into the left subclavian and then into the intercostal arteries which are located directly under the ribs, causing that "notch" like appearance.
