In this variation white plays to avoid the exchange of generals at all costs. The exchange of pieces tends to favor black for several reasons :
1. Black's rook and bishop tend to cover the squares on which pieces in hand might be dropped.
2. Pieces in hand favor the attacker, which is normally the player receiving the handicap.
3. Finally, it usually takes more skill to use the pieces on the board than the ones in hand; a good piece drop is usually obvious.
For these reasons, this line is rather tough to beat in practice; it's very hard to find the right plan with no piece in hand.
After 21... L1b
21... P7e 22 R4f (22 Px7e Gx7e helps white by letting the gold support ... P8f or ... P6f, and 22 P4d as above is not so convincing as there is no mate on 6b or drop on 7e here) S4b (to avoid a silver exchange) 23 P4d
23... S23a (23... Px4d 24 Bx4d K7b 25 P*4c! Gx4c 26 B5c+! P*4e 27 Rx4e P*4d 28 +Bx6d Px4e 29 G*6c K8a 30 +Bx7c R*9b 31 N*8d with an imminent mate) 24 Px4c+
24... Sx4c (24... Gx4c 25 P*4d G5c 26 N4e G5b 27 P4c+! Gx4c 28 P*4d G3d 29 Sx3d Px3d 30 P4c+ Sx4c 31 G*5c forking) 25 S4d (aiming for an exchange) S5b (White is consistent and avoids the exchange) 26 N4e
26... S4b (Centralization, just as in chess. 26... P*4b 27 N5c+ leads to promoting the bishop, while 26... P*4c is met by 27 N5c+ Sx5c 28 Sx4c+ leading to promoting the rook) 27 P*4c Gx4c (If either silver takes, 28 N5c+ wins material) 28 Sx4c+ S4x4c (28... S5x4c 29 G*5c leads to a bishop promotion) 29 Nx3c+ P*4d 30 +Nx4c Sx4c 31 Bx4d!
(Trading bishop for general to promote the rook is perhaps the most common motif of this handicap) P*4e (31... Sx4d 32 Rx4d K5b 33 S*4b is deadly) 32 Rx4d S*3d 33 G*6b! K7d 34 B5c+!! Sx4e 35 S*6c
with a decisive attack. In general I think this variation is one of white’s best, as some of the positions where black promotes at the cost of a knight may not be trivial to win.