The reported "heart beat" for the PASCO PASPORT Heart Rate Sensor - PS-2105 is a DC-filtered and offset value proportional to the amount of 940 nm radiation that is emitted by an infrared LED on one side of the ear clip and detected by a photodetector on the opposite side of the ear clip. The 940 nm frequency was chosen so as to maximize absorption by oxyhemoglobin and minimize absorption by other skin.
The intensity of radiation reaching the photodetector I is related to the molar absorptivity of hemoglobin e, the path length b, and the concentration of hemoglobin c by Beer's Law:
I = I0 e -ebc.
However, the signal is also related to the angle between the emitter and detector a, which may also change significantly due to a change in blood pressure. One's heart beat results in a change in b, c, and a, hence there is a changing signal related to the heart beat. When there is no change in these parameters, then the "heart beat" signal goes to the offset value (~87).
The heart beat signal, therefore, does not have a simple physical interpretation, but should change in amplitude according to the frequency of one's heart beat, provided that the subject is not moving around, which would result in an erroneous signal due to the change in angle between the emitter and receiver.
Although the "heart beat" signal does have some similarities to the EKG (electrocardiogram) signal, the "heart beat" measures an entirely different quantity from what is measured by an EKG sensor, which attempts to measure the electrical signals generated by the heart. Careful inspection reveals that certain EKG features are entirely absent from the heart rate sensor data.