Trying to limit the speed of a secondary esc/motor

[Jakyat]

Looking for help, or opinions. Im running a timbuk2 esc/18.5 turn crawler combo to run an auxiliary function on my truck. I currently have it drawing main power off the battery,and drawing signal from an aux in my rx(as per the friendly advice of "lil giant") my question; if I treat the esc like a signal, and I adjust it so it only travels 40% both directions, it slows my motor rpm down. Will this hurt my esc/ motor? What are my other options to slow this motor down?