Rule 3 permits a moving hamzah to be omitted when the preceding letter is quiescent, provided the hamzah's vowel is first transferred to that letter.
يَسْأَلُ → يَسَلُ
In the first form, sīn is quiescent and hamzah bears fatḥah. The fatḥah transfers to sīn, then hamzah is omitted.
This is vowel transfer followed by elision. Simply deleting the hamzah without accounting for its vowel would give an incorrect morphological explanation.
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Rule 3: Vowel Transfer and Hamzah Elision
Rules of Hamzated Forms: Lightening, Substitution, Elision and Short Paradigms
Rule 3 permits a moving hamzah to be omitted when the preceding letter is quiescent, provided the hamzah's vowel is first transferred to that letter.
يَسْأَلُ → يَسَلُ
In the first form, sīn is quiescent and hamzah bears fatḥah. The fatḥah transfers to sīn, then hamzah is omitted.
This is vowel transfer followed by elision. Simply deleting the hamzah without accounting for its vowel would give an incorrect morphological explanation.