Perhaps compare with use of a piece of very long, or indefinitely stretchable string, tied
to the lowest left-most pin, stretched vertically upwards on the left side of any others
with the same x-coordinate, till the loose end is higher than the highest pin. When the
string is pulled to the right until it is further to the right than the rightmost pin it
can be pulled down until it is further down than the lowest pin. It can then be moved left
until it is  below the initial (leftmost-lowest) pin and moved up to that pin. If pulled
tight it will form the convex hull. This takes a single, continuous motion.

compare:

http://en.wikipedia.org/wiki/Gift_wrapping_algorithm