Despite what I posted above, I'd also agree with that tactic, but in retrospect I guess it depends on the situation.
In my arena situation above just stopping and driving forward isn't a great option as it'll often create a traffic jam of other riders behind the problem horse. That's especially problematic if it's a mixed lesson with some less experienced riders. Circling into and away from the concern is compromise that allows traffic flow to continue.
On a trail ride I'd do the "Cut this crap out and get your butt in a forward gear" approach, however.
I think a lesson situation, where a bunch of inexperienced horses are on the rail together, is an accident waiting to happen!
I'm still getting Charlie over a bad experience, when she was run into, by another horse in the warmup
The warmup arena was very crowded, and I was loping her nicely on the rail, when the horse ahead of me balked, then ran backwards. I could not get out of the way fast enough, and that horse slammed into Charlie's flank, with the rider hooking my horse with her spurs.
Charlie leaped sideways and threw three huge bucks ,a s she made it from one side wall to the other. Luckily, we missed hitting anyone, and she did stop on a loud 'whoa'
Went back in and won a small pleasure class, but when we went in another one that was crowded, she freaked when a horse came up on her
It has taken me along time to get her over that.
Green horses/riders, should be worked individually in small groups , off of the rail.
I never train on the rail. I do my schooling in the middle of the arena, and the rail is then a good place to be, where you can drape those reins and let the horse relax