I started reading this thread and wondered why some were saying that the IF was the PU's responsibility and that the BU should be quiet.
I don't find anything like that in the softball mechanics manual.
The umpire's manual for baseball is formatted quite differently... and says several things different that what was being said here. The mechanics ought to be quite similar.
Here are some of the things I noted:
IF signal is "right hand raised overhead, with your index finger pointing at the ball."
"Don't be in a hurry to call the infield fly; make your call as the ball starts downward."
"Usually the plate umpire calls the infield fly first. This doesn't excuse a base umpire from making the call."
"When your partner calls the infield fly, regardless of how you see or how you feel about his judgement, you have to go along."
tootles!
