The WHEN clause contains a predicate which is evaluated first. If the predicate is evaluated to "true", then the trigger body is executed. Otherwise, the body is not executed. Essentually, this syntax is a left-over from the original implementation of triggers in DB2. IF statements achieve the same these days.
And as usual, have a look at the DB2 manuals because they explain the syntax and their semantics (although it is not always simple to find the right place):
http://publib.boulder.ibm.com/infoce...n/r0000931.htm