“Fake it” split function automatically adjusts your Tx VFO frequency on the radio to keep your actual transmit tones between 1500Hz and 2000Hz. Keeping your Tx tones above 1500Hz ensures that *if* you generate any audio harmonics that they will above 3000Hz and thus outside of the transmitter filter so that they will not be transmitted. Of course, having WSJT-x shift your VFO frequency requires that you have CI-V or CAT enabled.

If you don’t have CI-V or CAT then it is best to stay above 1500Hz on transmit to avoid the harmonic situation.