"Just lead the horse in holding treats in your hand." "Feed your horse in the trailer." Or force the horse in with pulling or pushing. ...
....With the second method, you have to have good timing of releasing the pressure or it doesn't work very well. DA's method is a little more lenient and simple. Outside = work and inside = rest. The horse makes the choice of whether to work or not. It's not a punishment for not loading. It's just having to work.
I think that if you don't have good timing & understanding of negative reinforcement(release of pressure on Right behaviour for eg) with ANY method, you're making life harder for yourself & your horse. And because horses learn from *instant* feedback, I don't believe DA's method is any more 'lenient', as it entails stopping asking for the horse to enter the trailer(so negative reinforcement for their resistance) before doing something totally different - lunging, before being asked to go back in the trailer, which is often only reinforced if/when the horse is all the way in, which may be way too much for some horses at that point in training, which if they're afraid, is not 'rest & reinforcement' at all, because they're so stressed about being in there.
Not at all saying it can't/doesn't work, but that in many cases, the horse is forced to just choose between 2 evils. I prefer to teach my horses to have no fear of the trailer and to learn that going in when I ask is a Good Thing, not just the lesser evil.
I also have to say 'work' outside for that purpose is most definitely punishment. That is precisely what it is - something you're doing that's unpleasant/undesireable, in order to be able to negatively reinforce(stop the unpleasantness) when the horse does the Right behaviour. *Not at all saying punishment is necessarily Bad either, just that's the definition of it. I personally train my horses to enjoy their 'work' so therefore I don't like to use things like lunging as punishment.