Forms from the same root show the limited scope of Rule 5:
Word / formMeaning / explanation
كُلْeat!; active second-person masculine singular imperative.
لَا تَأْكُلْdo not eat!; active prohibition.
يَأْكُلُhe eats; active imperfect.
Hamzah is dropped in the specified imperative, but it remains in the prohibition and the imperfect. The rule does not remove hamzah from every derivative of أ ك ل.