It's the magic tool when it comes to loading horses, I swear by it. All my horses, and all the horses I've ever had for a decent period of time, self load into a straight load or angle load and because I don't hesitate to use rope when needed.
Now before everyone gets up in arms, I don't hit my horse with it, I don't pull them on, they don't get tangled, and it is not stressful. Due to appropriate use of rope, I've never had to even think about using whips, force, aggression to load a horse and we have never failed to get a horse loaded.
Rope takes the onus off the handler.
My float loading motto: Act like you have all day, and it will take 5 minutes. Act like you have 5 minutes, and it will take all day.
Our way of doing it: The rope is tied to one side of the trailer at the edge and held my my dad on the other side. If there are two people one will hold each end of the rope, but normally it's just me and dad.
The rope is used as a barrier. We lift the rope so it sits just under the rump and above the hocks and the horse can feel it. There is minial pressure, unless the horse backs up, and then the pressure increases. While the horse is standing looking into the float or moving forward, it just sits there. The rope gives the horse no option to run out to the sides, and a lessened option to run backward. While you can't hold a horse with a rope, the pressure plays on their mind. They don't know you aren't strong enough to hold them.
A big mistake a lot of people make is trying to pull the horse on to the trailer. Most often, this causes the horse to throw up their head, which is dangerous as it can hit the roof of the float.
We always give the horse a chance to load calmy without the rope. If they resufse twice, the rope comes out. After they have loaded with the rope about 3 times, they never need it again.
I'm sure it won't work for every horse, but it has always, 100% worked for us.