[Caleb Rian]

The gibberish is working out your natural vowels and where they should sit in the melody. I find it always the best foundation for a lyric.

[AnitaFox]

This is something I have recently started doing as I have been putting more time into practicing topline writing as I'm not a musician.   
I usually write with a lyric to a melody in my head but have no way of getting it out.......very frustrating.
I'm finding the gibberish approach really useful.  I've watched/listened to a few clips of Sia doing it very well.
Cheers
A