In Shakespeare's Macbeth, why is Macduff's plan effective?
I assume you are referring to Macduff's plan to leave Scotland at his earliest convenience and seek assistance for Macbeth's overthrow.
The plan is effective in the sense that firstly, Macduff, in leaving in such a hurry, denies Macbeth the opportunity to assassinate him as he had done with Banquo. Macbeth has been aware of Macduff's sentiments and knows where his loyalties lie and wants him dead. In fact, the murderers who kill Macduff's entire family come looking for him for, on their arrival, the first murderer asks Lady Macduff, "Where is your husband?"
The murder of Macduff's entire family and his servants indicates how ruthless and bloodthirsty the tyrant has become. With his rushed escape Macduff had, essentially, abandoned his family and left them vulnerable and open to Macbeth's malice. In this regard, the plan was not entirely effective. In his defense, though, one can argue that he had no choice but to leave. If he had tarried, he would have been killed as well. His escape ensured that he would be able to acquire support for what has become a desperate cause—the survival of his beloved Scotland.
Macduff meets up with Malcolm in England, where he is informed that Edward, the English king, has promised support for their cause in the form of "goodly thousands"—experienced and battle-hardened soldiers. Macduff also later learns that Siward, "with ten thousand warlike men, / Already at a point, was setting forth."
He and Malcolm eventually meet up with the troops as they approach Macbeth's castle from Birnam wood. They later overrun his castle, and Macduff is brought face to face with the evil tyrant. He kills him in a sword fight and decapitates him. The bloody tyrant has come to an ignominious end, and Macduff has had his revenge.