(;
C[Black plays.

Source:
    In the Beginning, p. 139
    Ikuro Ishigure, Ishi Press]
AB[cd][dd][df][ee][fe][hd][dp][qd][ol][mm][mo][mp][np][oo][oq]
AW[oc][ed][ec][gd][ge][hc][de][pn][on][qp][ql][nq][mq][lp][kq]
(;
B[iq]
C[Essential.  Black keeps White's lower group weak so as to lessen the pressure on Black's own weak lower right side group.]
(;
W[ln]
C[White must first strengthen the weak group.];
B[nn]
C[But then Black strengthens his weak group.];
W[cn]
C[Then White can invade the left side.];
B[cp];W[ck];B[io]
C[But now Black can go back to attack White's weak group while making territory.];
W[jn];B[ci]
C[Now Black can attack the invading group to again make territory.]
)
    (;
    W[ck]
    C[White does not have time to invade the left side immediately.];
    B[jo]
    C[Black seals off the lower White group from the center.];
    W[or]
    C[And White must extend along the bottom to make life.];
    B[pq];W[pr];B[qq]
    C[White has made small life at the bottom, but Black now has weakened White's right side group.]
    )
)
    (;
    B[dk]
    C[Trying to make left side territory immediately lets White off the hook.];
    W[hq]
    LB[nn:A]
    C[White's extension here is a huge move.  White turns his weak group into a strong group while making territory.  Now White can aim at attacking Black's weak lower right group, e.g., with A.  In addition, with no weak groups, White can easily invade Black's left side at multiple points.]
    )
)

