Inhibiting the Janus kinase (JAK) family of enzymes found in hair follicles awakens the follicles out of their resting phase and promotes hair growth. (Hair follicles do not continuously grow hair, but instead work in cycles of growth and rest.)

JAK inhibitors are applied topically onto the skin and begin to work after a matter of days.
Accelerated growth has only been seen in patients without baldness so far, but it is possible that JAK inhibitors could cure baldness. Tests are currently under way on bald patients to see if it works.

http://www.sciencedaily.com/releases/2015/10/151023174914.htm