Update[symbol] tells Mathematica that hidden changes have been
made which could affect values associated with a symbol. Update[
] specifies that the value of any symbol could be affected.
Update manipulates internal optimization features of
Mathematica. It should not need to be called except under
special circumstances that rarely occur in practice. One special
circumstance is that changes in the value of one symbol can
affect the value of another symbol by changing the outcome of
Condition tests. In such cases, you may need to use Update on
the symbol you think may be affected. Using Update will never
give you incorrect results, although it will slow down the
operation of the system. See Section 2.6.13. New in Version 1.