In another embodiment, a method of drilling a wellbore includes running a drilling assembly into the wellbore through a casing string, the drilling assembly comprising a tubular string, an underreamer, and a drill bit; injecting drilling fluid through the tubular string and rotating the drill bit, wherein the underreamer remains locked in the retracted position; sending an instruction signal to the underreamer via modulation of a rotational speed of the drilling assembly or modulation of a drilling fluid pressure, thereby extending the underreamers; and reaming the wellbore using the extended underreamer.