slowly turn the TUNING dial from about 900 to 700 on the dial and notice how the tone at the receiver output changes.
this is because the local osillator frequency is being adjusted.
this has the effect of changing the frequency of the IF frequency.
Now,if the IF frequency is changed and the BFO frequency remains constant,the resuktant 'difference' frequency is changed.
this 'difference' frequency is the audio output signal that we are listening to.