What happans when we set spectrum analyser to zero span?

Plz any body correct me..
Am i right as its gives the complete time domain signal...as it doesn't sweeps and stays in a central frequency.

It acts like a power detector with an RF bandpass filter in front of it, and a analog lowpass filter after it. The bandwidth of the bandpass is ~ the RBW setting. The Lowpass filtering is set by the VBW. The center frequency of the virtual banpass filter does not sweep, but stays at what you programmed the spectrum analyzer for as a center frequency.

BW-->0 "Time Domain including DC"
You will see Power vs Time...

If your power isn't changed by time, you see of course a straight line at power level.