Some engine control heads feature an integral brake or friction mechanism for the throttle cable to prevent cable "drift" or "creep" when the return spring on the engine tends to return the throttle control to idle when you let go of it.
This mechanism keeps the throttle lever where you leave it.

An alternative, but not as good, solution for "cable drift" is to install this Throttle Cable Brake.
This is a simple screw clamp, installed anywhere along the length of the throttle cable, which crushes the casing against the cable, thereby increasing friction within the cable.