This my friends, is called the Six Minute Warm Up.
Some of the illustrators I know have been doing this. Many of them went to the NE SCBWI conference and attended a workshop, Character Design 101, taught by Kelly Light in which this was included. Unfortunately I wasn’t there, but I hear the class was fabulous and she cooks a mean chocolate chip cookie!
You might want to give this a try… it’s fun.