In an attempt to get my girls to drive off the rubber we do a drill where I stand in front of them, put our hands together and I have them push off with their drive foot/leg. The goal is to get for a full straight leg drive along with foot extension. We do this up and down the gym a few times once a week. That being said, I've noticed some of the elite pitchers also have an ever so slight twitch off their drive foot in an attempt to set the hips up for early open. A smidge of an angle on the drive foot shouldn't be a deal breaker, too many of the top pitchers do it. Strong drive, full extension of the leg and foot is what your looking for. Watch Monica's drive leg and foot. Both achieve full extension (explosion) off the "rubber" while still having a bit of an angle. Someone mentioned a while back that if they don't do that (full extension) they're just stepping off the rubber not exploding. Explode off that rubber.
She took a six inch step off the rubber prior to the start of the drive. She ever get called for it?