Calculus of indications: Why does $\overline{a \; b |}$ gives us the connective for the Sheffer stroke?

Question

I'm reading Williams "Logic and Integer Programming", here:

Question: I think I understand why $\overline{a |}$ gives the truth table given in there but why does $\overline{a \; b |}$ gives us the connective for the Sheffer stroke?