Before I lived in Japan, I had never really come across side reins. When I did, I think I might have done a post on here about them, and some people said that you shouldn't ride in them, it knocks the horse off balance, you should only lunge in them etc.
Now, the place where I worked was a dressage yard that used side reins pretty much all of the time for riders. They weren't hugely tight and the horses didn't overbend, but they were a standard part of the tacking up process there. I never found any problems riding with them. I rode Northy without them several times and didn't notice all that much difference in him, other than obviously he didn't bend as much. The only 'strange' experience I had of them was on a TB that I was riding one day, who was perfect and didn't put a foot wrong the whole lesson. At the end of the lesson, the side reins were removed and I walked her round to cool off, and she started acting like a complete idiot, spooking at everything and anything and jogging around.
I mean, I still don't know enough about them to say whether they're right or wrong, but that's my experience with them!
Whether or not you agree with these artificial aids, the correct use of them is as follows:
Side reins are used for lunging attached from the bit to the girth straps after the horse has been warmed up and removed before cooling or leading the horse. They can be used for teaching a rider on the lunge but only on a balanced horse, with a knowledgeable trainer and again only after warming up without. They are also used for long reining crossed at the wither and attached to the saddle ds, or to roller rings at the position of saddle ds, so the horse cannot poke his nose to bear forward on the reins and tank off. In lunging you can also use Vienna reins which sit in the same position as draw reins, but have buckles to attach to the saddle ds or equivalent roller position.
It is inappropriate to use side reins or vienna reins for riding as the rein is not adjustable during riding so the horse cannot use the neck correctly to help balance the rider. This is very dangerous if the horse trips or stumbles. For riding, draw reins or running reins* are used for this purpose as the rider can release them as necessary.
*nb Draw reins and running reins are the same piece of kit but draw reins are attached to the girth between horse's front legs and running reins are attached at the girth strap or equivalent height roller rings like side reins.
Like most things some people use them ‘incorrectly or inappropriately’ and some top riders, training yards and trainers advocate 'incorrect or inappropriate' use in specific circumstances, but the above is the accepted correct or appropriate usage.
__________________ "I have had many teachers in my life, but the best have all been horses"