Succi's Boundary condition

Hello guys, I’m reading Succi’s book on the sliding walls (p.87), where he listed

q=frac{rho_wu_w}{2(f_2-f_4)} (6.43)

why he said, … set rho_w rho/6.0, since only four populations f2, f4, f6, f8 are involved…, ?

here, all of the f2, f4, f6, f8 point to corner (fig. 6.5, p.83), should f2+f4+f6+f8=4*(1/36)=1/9?

Is this the Zou/He boundary condition? Can you give us a short summary of the “sliding walls algorithm”?