I recommend to patients for panic attacks and/or anxiety

The visual imagery helps some people focus who have difficulty initially doing mindful breathing. Patients I work with report watching the in-out breathing takes their focus off their panic. Decreasing the breathing rate to 3.5 helps them get control of their shallow rapid breathing, which also helps slow rapid heart beat. Ideally, after initial success with this, the patient should wean themselves off of an external support and transition to internalizing mindful breathing. Highly recommended for patients with smartphones or Macs. It is available for smartphones other than iPhones also.

Elsbeau about Breathing Zone