The knob on the FX24D is used to control drop speed. It is used on some of the YM models also and some it is not. No way I could give a list of which is and isn't. However, I do believe if your control valve is visible under the seat with the little T handle then yes, that controls drop speed and round knob is used only for lockout. "But", if the valve is enclosed it is very likely the round knob is used to control drop speed as well as lockout. Hope I made some sense.