She is making a strong effort to reach as far down to the bucket as she can reducing her chances of a miss. She has positioned her pony well and in fact is slightly “bending” the bucket which gets her hand even closer: When I teach the dunk, I do recommend that the riders begin to steer their ponies off to the right as they are dunking. Not only does this help place you in the right position for the dismount coming up, but as you can see, helps with the dunk as well.

One other thing worth noting is the fact that the rider is on a mount that fits her size. In this example, this is clearly an advantage.