How to check spectrum spread in FT-710?

I have been hearing my friends giving spectrum spread reports when other friends were testing homebrew linear amplifiers. Yesterday VU2MIE, who is also using an FT-710 radio told me how to check it and here it is. You can see two strong signals intermittently while they are talking. In one signal the spectrum is confined to the dense bar while in the other it is spreading both above and below the central frequency of LSB mode. When the other station gave the report, input signal to the linear amplifier was reduced by the operator and signal spread became normal. You may note that ‘CENTER’ is selected in the left lower corner. Usually I listen with ‘FIX’ there so that the band is shown from 7 to 7.2 MHz. When ‘CENTER’ is chosen, the tuned frequency will be shown in center (7.138 MHz in this case). Next option for selection is ‘CURSOR’. When ‘CENTER’ is selected, spectral spread to either side is shown in steps of 2kHz (positive to one side and negative to other side).