Some calves are bulky because of muscle, not fat. Genetics, height, a lot of walking in heels, or an athletic build can all give the gastrocnemius, the muscle that forms the curve at the back of the calf, more size than you want. Standard fat-reduction methods do nothing here, because the bulk is muscle. Calf slimming relaxes that muscle with botulinum toxin, and over the following weeks it thins, softening the curve and creating a more tapered line through the lower leg.
I want to be clear about two things up front. This works on muscle, so it is not for soft or fatty calves. And I will tell you honestly at your consultation what change is realistic for your calves.

Calf Slimming
For calves that are large because of muscle, from genetics or an athletic build, rather than fat. Not suitable for athletes, runners, dancers, or anyone who depends on calf strength for their activity, because the treatment relaxes the muscle.
Explained at your consultation. Any change is gradual, and results vary from person to person.
Most people return to their day right away. Some mild muscle aching, cramping, or bruising at the injection sites can happen and settles on its own.
It is a deeper injection than a facial area, placed at several points across the muscle. I go through what to expect at your consultation.
How long it lasts varies from person to person, and repeat sessions are needed to keep any change. The gastrocnemius is a large muscle and needs more product than a facial area, dosed to your individual calf.