How the extend feature works in the cascade
Sass' Silent Placeholder function works the same as normal CSS extending in the cascade
This example here illustrates how that Sass will extend a silent selector in the cascade the same way that a normal selector would. The only difference is that silent placehodlers are not processed into CSS until they are called into a selector that is processed into CSS.
Play with this in SassMeister to see how the processed CSS changes between the different selectors. http://sassmeister.com/gist/4512325