How does it work?

 

From the PulsePoint website: 

Where adopted, PulsePoint Respond empowers everyday citizens to provide life‐saving assistance to victims of sudden cardiac arrest.

App users who have indicated they are trained in cardiopulmonary resuscitation (CPR) and willing to assist in case of an emergency can be notified if someone nearby is having a cardiac emergency and may require CPR.

If the cardiac emergency is in a public place, the location-aware application will alert users in the vicinity of the need for CPR simultaneous with the dispatch of advanced medical care. The application also directs these potential rescuers to the exact location of the closest Automated External Defibrillator (AED).

Source: https://www.pulsepoint.org/pulsepoint-respond/