What is significant about زَلْزَلَ and وَسْوَسَ here?

Final Characteristics of Derived Forms: Ifʿīlāl, Ifʿilāl, Faʿlala, Tafaʿlala, and Related Patterns

• Only defective triliteral verbs
• Only instrument nouns
• Imperatives of Form IV
• Examples of repeated consonants in quadriliteral forms