From an umpiring standpoint ...
-IFF can be called even if it's not signaled, if the game state and then the playing action merits it
-However, from proper mechanics at least as I've been told, it should be signaled (visually and verbally) by the plate umpire once identified. As a base umpire, I will give a visual signal if I have a hit ball meeting infield fly consideration. Then as a plate umpire, if I see my base umpire give a visual signal, I am going to "piggy back" that with a verbal (and visual)
-When the IFF is not signaled, and it should be, it creates confusion. Then again, IFF situation properly called by umpire can become confusing as well if ball hits the ground because players start to panic.
-IFF can be called even if it's not signaled, if the game state and then the playing action merits it
-However, from proper mechanics at least as I've been told, it should be signaled (visually and verbally) by the plate umpire once identified. As a base umpire, I will give a visual signal if I have a hit ball meeting infield fly consideration. Then as a plate umpire, if I see my base umpire give a visual signal, I am going to "piggy back" that with a verbal (and visual)
-When the IFF is not signaled, and it should be, it creates confusion. Then again, IFF situation properly called by umpire can become confusing as well if ball hits the ground because players start to panic.