Commonly referred to as Potassium Alum, in the science world, it is a chemical compound that acts as an antiseptic and also as a blood coagulant (stops bleeding, by thickening the blood).
The general practice is to rub the block (preferably “chilled”) over a freshly shaved face, to stop any potential bleeding or irritation.
Quick caution: For some men it may deliver a brief sting. But believe me, whether it stings or not… it’s working!
Give it a try!