The effect that is observed in these Biopac experiments is called
respiratory sinus arrhythmia. It is described on p. 384 of Berne and
Levy's 4th edition Physiology text. I have found it to be very
reproducible in this Biopac experiment if the students take care to keep
track of and record inspiration and expiration carefully and exaggerate
their breathing.
The heart rate increases during inspiration and decreases during
expiration. This is due to the change in intrathoracic pressure. During
inspiration intrathoracic pressure decreases and this aids venous return.
The increased venous return stretches the right atrium, setting off the
Bainbridge reflex (which is an increased heart rate upon stretching of the
atria).
In addition to this, apparently stretch receptors in the lungs may also
somehow result in a relflex increase in heart rate and there may be central
factors involved as well.
The change in voltage observed in the Biopac figure could simply be due to
movement of the electrodes during the repiration so that their positions
relative to the ECG vector have changed. I doubt that that is
reproducible, but the change in heart rate is almost always present.
I hope this helps. (Marieb also mentions the Bainbridge reflex on p. 707
5th edition.)