In this line white concentrates solely on making it hard for black to break in, rather than on doing anything constructive himself. In even game shogi such passive play is generally fatal, but in handicap games, especially larger handicaps, aggressive play by white is apt to result in a quick win for black, who will usually break in easily if white does not concentrate all his forces on defense. So lines like this are probably white's best practical chance.
After 21... L1b
21... S4b (!) 22 R4f S23a (This looks very passive, but it is not so simple to refute; it is similar to the preceding variation, in which ... P7e was played first) 23 P4d Px4d 24 Bx4d K7b
After 24... K7b
(White simply prevents any promotion or exchange) 25 N4e P*4c 26 B5c+ Sx5c 27 Nx5c+ (The knight's promotion compensates for the lost exchange)
27... B*7a! (27... S4b 28 S*4a G3a 29 S5b+) 28 S*6b Bx6b 29 +Nx6b Kx6b 30 B*8b.
Black will regain the sacrificed material and promote his bishop, and can continue his attack by P9e, aiming to promote a pawn on the 9th file. An improvement for white on this line by Kato Hifumi 9 Dan is to replace 18... P9d by ... P1d, so that the above plan for black will take more time. This may well be white’s best line against the crab castle system. At least he won't lose quickly.