Someone that appears attractive from a distance whom upon further inspection turns out to be totally hot often causing others to stop and marvel at their beauty and/or hotness.
The term is derived from the motions of which you would take when approaching a stop sign in a car. You slow down to check said person out and when you come to the conclusion that they are indeed worth the effort you come to a complete stop and approach.
A person whom from a distance appears attractive but upon further inspection turns out to be ugly.
The term is derived from the motion in which you would drive over a speed bump. You slow down to check out said person and upon discovering how unattractive they really are you resume speed and continue on course.
Two young men walking together -
Joe: Yo, that girl is phat to death.
Rick: Yeah, lemme catch up to her before I try to holla.
"Rick and Joe catch up and continue to walk past"