I LOVE Sally Swift, her books are amazing, and I am lucky enough to have a Centered Riding Coach!
One of my favorite exercises is when she ties yarn to the bit, and then that's all we have to control our horse (Obviously the real rein is still there, just tied in a knot). Its a great way to test if you are leaning on the reins - something I used to do horribly, and old habits die hard so I love this one to keep me in check!
This is something you have to do with a horse that really is already listening to the seat very well, otherwise you are going to break the yarn pretty quickly!
I have never heard of the exercise where you use yarn!
Sounds like an excellent exercise!
Thanks also to everyone who contributed!