Before I even start, this is a bad idea. Don't do it. Especially if your trainer can see you and think it's very clever and shows initiative. You've been warned!
Back when I was an h/j rider, I spent a fair bit of time with a dressage whip behind my back and in front of my elbows. This was to get my shoulders back and bring my elbows in close to my sides. I'm sure I'm not the only one that's gone through that particular type of torture.
Well, that's spiffy and all, but now I do the dressage thing and my elbows aren't supposed to be back that far. They should be at my sides, but not that far back. I've been struggling so much with getting them forward and still close to my sides. The phrase that currently works to remind me? "Squish your boobs!"
For the record, I'm a D cup. They stick out more than my rib cage on the sides and most sports bras make them flat, but they get wider. There's nothing I can do about it.
So what do I do when I'm not in a lesson so there's no one to yell about my boobs? I really should get one of these things.
But I'm perpetually horse poor, so I took the obvious, free option as inspired by my h/j days. I put my dressage whip in front of me, across my rib cage just below my bra, and behind my elbows. This forces my elbows to stay more forward, forces me to not hollow my back (I need some tummy to keep the whip from falling), and forces me to keep my elbows at my sides.
I wasn't sure it would work, but I managed to do my walk warm up and some of my trot work like that. My shoulders are not pleased with this new way of going, but it forced me to figure out how to get a contact with Theo without dropping my hands. Trainer A spotted my silliness and thought it was a genius idea. Guess who's going to have to start carrying a whip down to the ring again?
I'm a masochist.