WIth the above - it's an unwritten/unofficial code whereby drivers tell each other it's safe to pull back in after overtaking, and also a way of saying 'thanks' to each other.
Problem is that car drivers will go using it when they haven't a clue what it's all about. I've been out with my bloke in the past where car drivers have signalled to him to pull in and it's clear there just isn't enough room to do so. Not their fault, of course, they're just trying to be nice, but they often underestimate just how much room a 40ft artic needs to manoevre safely. If you're not sure, don't do it. Drivers do understand. As Vulcan says, truckers are nice people really - just misunderstood.
Perhaps we should have a 'be nice to a trucker today' day.