Intel Secure Key Entropy Source

Hello,

The following doc states the following in section 3.2.1.

https://software.intel.com/sites/default/files/managed/4d/91/DRNG_Software_Implementation_Guide_2.0.pdf

"The ES runs asynchronously on a self-timed circuit and uses thermal noise within the silicon to output a random stream of bits at the rate of 3 GHz. "

The doc doesn't describe how the entropy source works.  How are these features of the entropy source measured and how do they contribute to entropy?

Thanks,

Eric