The key to a good canter depart is a straight, connected horse and a quiet, precise cue that doesn't change his balance. This exercise slows both the horse's and rider's mind before a trot to canter transition, allowing for more clear communication in the actual aid.
If your horse finds lateral exercises more difficult in one direction vs the other (as many do), then they are not truly straight, or evenly laterally supple.
"I used to ride mostly Thoroughbreds. I don’t know if they’re more intelligent, but I find that they think for themselves a bit more than the warmbloods. With a warmblood I find I definitely have to do a lot more, and think a lot more for them.
In the sport of Eventing, ensuring that our horses are truly fit for our level of competition is one of the best ways to prevent unnecessary injuries.
"It is not dressage that is difficult, it is making it look easy that is difficult. Some horses are born with presence, while others you can train presence. Either way, dressage is about improving the paces." ~ Carl Hester