Nobody Stands Still For A Single Heartbeat

You cannot stand still. Not on a scale, not in bed, not for one beat of your heart.

Each heartbeat sends about 70 millilitres of blood up out of your heart and around the arch of your aorta, the main artery. The pressure differences that push that blood along also push back on the rest of you, so your whole body is rocked to and fro with every beat - a small movement one way, then a larger one the other way. The effect has a name, the ballistocardiogram, and it is large enough for a scale to show. In 1877, J. W. Gordon described watching the index of a weighing scale move in time with the pulse of a man standing on it. Today a force sensor under a bed can count heartbeats without touching the sleeper.

Sources, in the order the video makes the claims:
The whole body is rocked by each beat, and the force travels through pressure gradients in the ascending and descending aorta (Kim et al., Scientific Reports, 2016): https://pmc.ncbi.nlm.nih.gov/articles/PMC4977514/
The rocking has a fixed pattern - an early, smaller footward deflection (I wave), then the largest one headward (J wave) (Sadek, BCG review, 2018): https://arxiv.org/abs/1807.00951
Resting stroke volume, 60 to 100 mL in healthy adults - "about seventy" sits inside it (StatPearls): https://www.ncbi.nlm.nih.gov/books/NBK547686/
The scale that moved with the pulse - Gordon, J. W. (1877), "Certain molar movements of the human body produced by the circulation of the blood", Journal of Anatomy and Physiology 11, 533-536: https://pmc.ncbi.nlm.nih.gov/articles/PMC1309740/
A load cell under a bed detecting heartbeats with no skin contact, plus a summary of Gordon's observation (Albukhari et al., Sensors, 2019): https://pmc.ncbi.nlm.nih.gov/articles/PMC6470700/
Pulse read from the tiny movement of the head on ordinary video - the same reaction, seen by a camera (Balakrishnan, Durand & Guttag, CVPR 2013): https://people.csail.mit.edu/mrub/vidmag/papers/Balakrishnan_Detecting_Pulse_from_2013_CVPR_paper.pdf