After a few years away from this and I left out the most important part!!
After reaching the farthest distance (where form falls apart), have her start moving back towards the pitching rubber, again by 3 paces at a time. When she reaches the pitching rubber and throws from there, she will surprise you and herself by how much more energy she is putting into her pitches!
The progressive moving back is a gradual process to force her to throw harder. Moving back as she gradually shortens the distance, she should retain the energy of the motion at the longer distance.
For you diehards, try using your radar gun for a "before & after", and show her the results. Take an average reading from the pitching rubber. Take another average when she gets back to the rubber.
If you do this at the end of a pitching session, try using just 2 pitches at each spot. Or do this drill at the beginning of a session before she's tired. In our case, we typically did it AFTER the pitching session to force some stamina. REMEMBER GOOD FORM!!
I'm convinced this drill has contributed to DD being able to "pitch marathons" when needed. She set the record for season innings pitched last year as a junior in college. She had some arm soreness - muscular - but never has had any injury related to pitching. I'm sure that being in good physical shape contributes a lot too.
I'd be interested in anyone else's results (NOT the speed, just the percentage of increase in speed).