First, let me say that I hate the tight bases with stealing rule in softball. It is the worst rule in softball, with the look back rule being a close second. I understand there is no "stretch" with pick-offs in softball (I wish there was), so lead-offs are not possible. However, being called out for leaving early is just wrong - the punishment doesn't fit the crime. I think the runner should be forced to return to the base with a warning the first 1 or 2 times, then be called out if it happens a 2nd or 3rd time, similar to the false start in track.
Now, I was always the SB leader on my baseball team and reading the pitcher to get a good lead was an art form. Getting a good lead off/jump was crucial in a successful steal. So, having one foot behind the base was a completely foreign concept to me. In fact, I laughed at it the first time I saw it. DD is a speedster and always leads her team in SBs too. She has experimented with starting positions and has settled on having one foot behind the base. I'm still torn on if it makes a difference either way. I've timed her and noticed very little difference in her speed.
The challenge has been more on timing than foot position. The 1B typically positions herself so the base runner's view of the pitcher is obstructed and DD has gotten some bad jumps on straight steals. The 1B coach needs to recognize this and do a better job. So, I agree with Northball's advice that much of the burden falls on the 1B coach to help the runner and teach her to look for signs he/she may be seeing.
if the runner is timing and rocking/starting correctly, that back foot is on first stride on the release, no longer behind bag. the entire purpose of that is to have runner actually moving, with momentum, at release. as long as the lead foot is on the bag still, they can be moving, and that lead foot (which at release should actually no longer be lead foot if that makes sense) us using bag as a sprinter uses starting block.