As I was reading all of your comments, both here and on Facebook, I was reminded that many of you are not dressage riders. You are reiners, endurance riders, jumpers, or trail riders. Some of you don't even ride which means you might not always know what the heck I am talking about.
As defined by USDF, "This is a change of lead where the horse is brought back immediately into walk and, after 3 to 5 clearly defined walk steps, is restarted immediately into a canter on the opposite lead with no steps at the trot."
Basically, it's a canter to walk transition with three to five walk steps before cantering on the opposite lead.
What makes this movement so challenging, for us anyway, is that the canter has to be very collected so that the horse is carrying more weight on his hind. Carrying more weight on the hind end is hard for the horse. Think about doing squats. Now imagine holding an armload of something heavy and then squat. Not so easy. Not so easy for the horse either.
A tip Chemaine Hurtado, owner and trainer at Symphony Dressage Stables, shared with me was while on counter counter, prepare for the walk by straightening the bend. As the horse transitions to walk, do a shoulder in in the new direction to aid your horse in picking up the new lead.