Can someone explain part (b)?

[Robert E. Harrison]

8.3.1 SITUATION A: With R1, R2 and R3 on third, second and first bases,
respec tively, and (a) two outs or (b) one out, R1 attempts to steal home. With a
1-2 count, the pitch hits R1 while the ball is in the strike zone. RULING: In both
(a) and (b), ball becomes dead immediately and batter is out because of third
strike. In (a), no run is scored since batter became third out. In (b), all base runners
are awarded one base from where they were at time of pitch and R1 scores.
(5-1-1a, 6-1-4, 8-1-1 note 1, 9-1-1a)

[dash_riprock]

It's the rule. When a pitch strikes a runner (it will always be a runner from 3rd stealing on the pitch), the ball is dead and all runners move up one base. It is still a pitch, and it is what it is. In this case it is strike three, so the batter is out on strikes (and can never advance). If it's the 3rd out, no run can score. Otherwise, count the run.

[mbyron]

When a batted ball strikes a runner, the runner is (often) out. When a pitched ball strikes a runner, the ball is dead and other runners advance 1 base.

As dash said: it's just the rule. I expect it evolved to penalize pitchers who threw at the runner stealing home in order to send him back to 3B.

[MikeStrybel]

They nailed it. I recall this play being discussed a while ago here. At the time, an umpire insisted that it was a balk since the pitch never crossed the plate or foul lines. That sounded good until your note that the batter is called out on the strike, so it had to be a viable pitch. This is just another great example of why this game is amazing and we have to be on our toes.

Have a great season.