This device monitors pressure needed to cause transient digital arteriolar occlusion using MEMs sensors. It then uses an algorithm that uses this data along with pulse oximetry data to estimate the brachial arterial blood pressure.

In situations where it actually matters ( patients with abnormally low or high BP, patients with low peripheral perfusion, patients with peripheral vasoconstriction, critically ill patients ) this will be measuring something with little or no correlation with the brachial BP.

Theranos made similar promises...