Rāʾ at waqf is a final rāʾ made sākin by stopping. The book therefore looks behind it to determine heavy or light pronunciation.
First check the letter immediately before rāʾ. If that letter itself is unvoweled, further examination may be needed, including whether it is a sākin yāʾ. This is why a rule for an ordinary sākin rāʾ cannot simply be applied blindly to every word ending in rāʾ.
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Why a final rāʾ needs fresh attention at waqf
Tafkhīm and Tarqīq — Alif, Divine-Name Lām and Rāʾ
Rāʾ at waqf is a final rāʾ made sākin by stopping. The book therefore looks behind it to determine heavy or light pronunciation.
First check the letter immediately before rāʾ. If that letter itself is unvoweled, further examination may be needed, including whether it is a sākin yāʾ. This is why a rule for an ordinary sākin rāʾ cannot simply be applied blindly to every word ending in rāʾ.