The common feature explains the fitness of a word for its new meaning; the clue indicates that its literal meaning is not intended in that context. The concepts are related but analytically different.
In “ṭaghā al-māʾ,” water in the context prevents a literal reading of deliberate rebellion; the common feature is exceeding a bound. In “faṣdaʿ bimā tuʾmar,” the command provides the clue; the common feature is bringing a hidden matter into clear expression.
When asked for the nature of a common feature, the answer must derive from the relation between source and target meanings, not from the clue alone.