This example is pretty much beaten to death, but consider the Square/Rectangle or Circle/Ellipse possibility. Suppose you have a base class Rectangle that defines an object with a length and width. If you have a Square class that inherits the Rectangle class, it would have a rule in its setter/getter that would require that any change to length or width would alter its counterpart. These dimensional requirements strengthen pre-conditions because a rectangle substituted for a square would be missing these dimensional requirements. Suppose you reverse the inheritance so that a Rectangle inherits a Square, you would be weakening post conditions by relaxing the dimensional requirements to allow the Rectangle to behave independently.
However, if you were to remove the dimensional change capability, the substitution principle holds because if neither a Rectangle nor a Square may change dimensions, then they have equal pre and post conditions regardless of inheritance. Both have a length, both have a width, and neither can alter those values.
ref: Wikipedia - http://en.wikipedia.org/wiki/Liskov_substitution_principle