The sawtooth wave from the VCO-out connection can be converted to a pulse wave of any width. When the width is exactly 50% you get the distinctive hollow sound of the square wave. These waveforms allow you to create a LOT more sounds.
The idea behind this mod was to change the sawtooth into a pulse wave and then feed it into the AUX input jack. The main challenge was how to cut off the sawtooth wave connected internally in the Monotron so you could just hear the new pulse or square waves and not the sawtooth.
I angsted over this for a long time as said I didn't want to "break" the monotron. In the end I decided that removing a capacitor was necessary but would "break" the monotron i.e. it wont output any sound without the mod in place. You may not want to do this mod if you want to use your Monotron without this mod connected.