وَيُخْفَضُ بِلَامِ الِاسْتِغَاثَةِ، نَحْوَ: «يَا لَزَيْدٍ!».
Translation: In an appeal for help, the vocative noun takes a genitive form through lām, e.g. “Yā la-Zaydin!”.
Istighātha is a cry for assistance. The open-lām marks the appeal and changes the surface construction.