An unusual complement is one whose association with the verb is outside normal expectation and creates rhetorical force. In «أبكي دما», deleting «دما» removes the specific portrayal of grief that the speaker wants the audience to imagine.
ExpressionWhat the hearer receives
لو شئت لبكيتA hypothetical capacity or wish to weep, without the unusual manner
لو شئت أن أبكي دما لبكيتهThe extraordinary image of weeping blood
لو شاء لهداكمAn object recoverable from the consequence
The difference is not word count alone. Deletion suits an already evident relation; explicit naming can be necessary when the unusual content itself carries the force.