Ash-Shaykh Abdul-Muhsin al-Abbaad, may Allah preserve him, said:

أما العمامة وكيف يكون الإسبال فيها فالذي يبدو أنه مثلما ذكر بعض أهل العلم أنه الشيء الذي يكون خارجاً عن المعتاد أو عن المألوف بأن يطول طولاً فاحشاً كمن يسحب العمامة وراءه من فوق رأسه، فالإسبال في العمامة هو ما يكون فيه زيادة فاحشة كبيرة عن القدر المحتاج إليه.

As for the Imamah and how isbal can be found therein, then what is apparent is that it is similar to what some of the people of knowledge have mentioned. [It is] that it is [defined as] what exceeds the regular custom [of the people in the length of their imamah] or what the people are familiar with. Meaning, that it is awfully long, similar to a person who pulls his Imamah behind him from the top of his head. Thus, isbal in the Imamah is wherein there is an extra length which highly exceeds the required amount that is needed.

[Sharh Sunan Abi Dawud (19/459)]

Translated by

AbdulFattaah bin Uthman
Abu Fajr