Weather radars with turbulence detection have very stable frequency transmitters that allow the receiver section to see returns that differ in frequency caused by the doppler shift as precipitation or dust particles move towards or away from the aircraft. To avoid spurious returns, airspeed is fed into the radar to provide a speed range gate. Obviously the returns from such moving particles are much lower in energy to the main one from static targets hence turbulence only working at closer ranges. In the WX position, only the main (non-doppler) return is used. Both WX & WX+TURB may use other processing techniques to better show what's out there - exactly what seems to depend more on manufacturer than pure physics.