SmartLeash comes with a detachable wrist strap. To attach it to the leash, put it through the handle and then pass the large loop through the small loop. The small loop snugs up to the handle, the large loop goes around your wrist. The black slide allows you to snug it up around your wrist.
Besides the obvious reason for using a wrist strap - so you don't accidentally drop the leash and lose control of your dog - there's another very good reason: When you drop the leash, it scares the dog. Really. This will happen with any retractable leash - it makes a loud noise and if the leash isn't locked, it will start to rewind, effectively chasing your dog, which scares them more. This can cause your pet to start running, leash and all, with the leash following them. Really, it's not a happy scene! Please use the wrist strap!