Majaz mursal is figurative usage in which the relation between the customary sense and the intended sense is something other than similarity. The context lets the hearer move from the named thing to its closely related referent.
For example, a hand can be the means by which someone provides a favor, so the word “hand” may stand for the favor. The favor is not being described as looking like a hand; there is a causal association.
The decisive distinction from istiara is the kind of relation, not whether the expression sounds striking or elegant.