THE RUNNER: 7.05 (b)

[wpiced]

I have been wrestling with the second half of 7.05 (b) for some time now. So, what does it mean? "Each batter may,
... advance one base when:"

(b)... or when the batter hits a fair ball that touches another runner or the umpire before such ball has been touched by, or has passed a fielder, IF THE RUNNER IS FORCED TO ADVANCE: What?

First, I assume that "a fielder" can also be the pitcher in this case. Second, can anyone describe a play where the runner is FORCED TO ADVANCE and gets hit by a fair ball that hasn't passed a fielder?

I can only think of a bases loaded situation where R3 is trotting (forced) home and stumbless over a slow roller that is along the third base line.

Somehow, I think I am missing the point. Who knows the answer?