The ideas in this post largely come from Nicolas Collins' excellent book 'Handmade Electronic Music', which I would highly recommend.
The 4093 quad nand gate has four logical blocks, whereby each block has two inputs and one output. Each output is HIGH unless both inputs are HIGH, in which case the output goes LOW. This is summarised in the following table:
One nand gate can be used to create a stable oscillator, by first connecting one input to HIGH, and then feeding the output back to the other input of itself. Much in the same way as a 40106 oscillator, the frequency is determined by a resistor and a capacitor.
The output of this first nand gate, which is oscillating, can then be used to feed into one of the inputs of a second nand gate. The output of the second nand gate is then connected back to the unused input of the second nand gate, which is also controlled in terms of frequency using a capacitor and a resistor.
In effect, the output of the first oscillator is determining whether or not the second oscillator is heard - thus modulating the amplitude.
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