Little League closed bases. Bases loaded, 1 out, defense in on the grass. Umpire in C position behind and between 3B and SS but in the back of the dirt. Hard hit ball between the two fielders and hits the umpire. Is this a live ball or is it umpire interference?
Live ball. The base umpire was behind the infielders.
On a 90' diamond, when the base umpire is working on the inside, in front of the infielders, then it could be umpire interference.
Think of it this way, the runner's job is to NOT get hit by a batted ball - the runner needs to avoid the ball.
Bases loaded, infield playing in, grounder towards F3.
R1 heads towards second (he is running behind the fielders who are playing in).
As I said, the runner needs to avoid being hit by the ball so what is the "exception"?
Well, as the runner is running "behind" F3 the runner can't see the ball (F3 is in the way).
So, if the ball goes "through/by" F3 and hits the runner behind him you really can't blame the runner - he couldn't adjust his running because he couldn't see the ball, so play on - no out.
Same scenario, gounder hit between F3 and F4.
The runner can see the ball (no infielder blocking his vision) so if the ball hits him he is out (even though the ball has "passed" the infielders).