Each side of the tone control exerts a fairly shallow filtering effect, given that each half is 6db/octave slope. Although the resistance of the 100k Tone pot does result in attenuation of whatever side the control is turned "away" from, there is still some content getting through. So, if one adds the appropriate resistance to the treble/highpass side, and lowers the corner frequency of that side, then as you turn from full treble to full bass, there will be a broader range of higher frequency content that still "sneaks" through. And the same thing going the other was with the bass content.
Think of it like being able to persuade the doorman/bouncer at a club that they can let your friend in along with you, even though everybody else has to stay out in line.
