It's the device you use when your heart goes into cardiac arrest to shock it back into a normal rhythm, or, as one of the guys I was teaching a class to referred to it as: "The shocky-hearty-box thing."
Es un dispositivo que se usa cuando tu corazón sufre un paro cardíaco para devolverlo a un ritmo normal. O como uno de mis alumnos lo definió: "La cosa en forma de caja que da descargas al corazón".