This is also one of those things that is umpire's judgement. You're not going to have instant replay available to see "yes, his foot was on the ground but 0.5 microns short of the line so he didn't cross it before he was tagged". I've played for years and umpired nearly as long at this point and one thing that's a certainty - softball players LOVE to argue. I'm sure we'll argue about that, too.