what is the hand signal to let a trucker know he has a loose load?

by andy
(roanoke)

I recently attended a driving course in Ohio and the instructor told us a hand signal that can be used to let a trucker know they have a loose load or something may be wrong with it.

I don't remember what it was and today we went past a semi that had something wrong in the back where he couldn't see and i didn't know how to let him know?

what is the gesture or signal that can be given to a trucker to let them know?