Rooting hormone is a class of auxins that promote root initiation. Naturally occurring auxins are found in plants. While a cutting is put in a dry environment, the auxin concentration increases along with water evaporation. Once it reaches a certain threshold level, roots start to initiate. In horticulture, synthetic auxins like IBA and NAA are commonly applied to stimulate root initiation.
Haworthias hold significant water and natural auxin in bodies. While drying, a Haworthia cutting can easily reach the threshold level of initiating roots. Actually the most common cause which prohibits Haworthias from rooting is microbial infection and the consequent hormone imbalance. Therefore when propagating from cuttings, infection prevention should be taken priority over hormonal regulation.
There are powder and liquid hormone formulations. Liquid is recommended since it is easier to dilute precisely. Excessive high auxin concentration actually inhibits root elongation. The ideal auxin concentration for Haworthias is ~0.5% (500ppm). For dilution, use 91% rubbing alcohol (Isopropyl) as the solvent. Alcohol assists auxins to permeate plant tissues, and also kills most microbes on the wound surface. For application, dip the base of a fresh cutting into the solution for 3~5 seconds. And while still wet, dip it into sulfur powder to form a coating around the wound. Let the cutting sit out in a dry, warm and shaded place, and wait for it to form a callus before planting. This should take a few days to a week. Sulfur coating works as a fungicide during this period.