Should not leave welts.
The only time a whip should raise a welt is if it is used to save the person holding the whip from being hurt. IOW's, a horse backing up and kicking or running at a person biting and striking.
A lunging session is not the place to provoke a horse to kick or bite.. it is a session to teach a horse how to track up, move on a circle, build his ring of muscles and reach under himself with his hind legs.
Tell me, when you see this "trainer" lunging the horse, is the horse tacked up or in a lunging surcingle with side reins and a bridle and a lunging cavesson (a lunging cavesson has a heavy nose band with an attachment for the line that is on the top of the nose). OR is this 'trainer' lunging him with a halter on?
The first is a training set up and the horse is learning. The second is an amateur set up and the horse is only learning to run in a circle and NOTHING else.
There is something about the outside of a horse that is good for the inside of a man. ~Winston Churchill
) Dinosaur Horse Trainer