The main example he used to show the thought process in designing the shell was the use case of simply dialing a number. How does a blind person do this accurately on a touch screen? How does he know where the numbers are?
The answer comes from the idea that the numbers don't always have to be in the same place; they only need to always have the same relative position. So if you say that the digit 5 will always be located the first place you put your finger (or thumb), you can then slide your finger around to the relative position of the other digits, laid out as they normally are on a phone. Audio and tactile feedback is given as you slide over the numbers, and the digit is spoken aloud once selected.
The video below shows the dialer in action (skip to about 1:50 if you don't want to see the introduction).
The cool thing is that this is useful for everyone, not just blind people. If you're walking along the street and need to dial your phone, you generally want to continue looking where you're walking. Otherwise, you'll be like the person who apparently walked right into T.V. on the street when not paying attention. He laughs about that, since he's the one who's blind, yet he's the one being walked into! ;)