throw-if-tag-found checks whether it can find the catch tag catch-tag by using find-throw-tag. If it finds catch-tag it throws to catch-tag the value(s) of evaluating result-form. Otherwise
nil, without evaluating result-form.
The throwing operation is done by a normal throw. Therefore the only the difference between this and
cl:throw is when the tag is not found. In this case,
cl:throw would evaluate the result form and then give an error, but
throw-if-tag-found simply returns nil.
LispWorks User Guide and Reference Manual - 13 Feb 2015