Frankly, some of the suggestions you've been offered scare me.
Be careful with the "Instructo-swing" device. Every model produced up until this year has had the "rails" that the batter swings through oriented on a downward path.
A downward swing is not recommended, is in direct conflict to what is being taught by elite softball coaches at the NCAA and Olympic level and does not emulate a high-level swing.
And, if you view video of Ken Griffey's swing, he most definitely does not swing downward through the hitting zone.
This training aid has been debated for years. I see from their website that the latest 2006 model available for sale has been redesigned. The device now features adjustable bars, allowing them to be tilted at an upward angle.
Strange that a product that has been on the market a dozen years, and claimed to enforce the "correct" swing mechanics, now is adjustable to cause the batter to swing with the exact opposite swing path that the product had always enforced.
The two-tee drill, with the back tee being higher will promote the same downward swing, which I do not recommend.
It's hard to diagnose a hitter based on a short discussion board post. I will offer you some general suggestions and you can decide which ones apply or which to try in practice.
- The hands themselves are not independent entities. They do not move on their own, rather they are moved BY the arms and shoulders. So when you talk about hand movement, something further up the chain is what's really doing the moving.
One cause of rear shoulder dipping, and a resultant hand dip, is overstriding. Does your hitter employ a long stride before hitting the ball?
- If you focus and drill on what the shoulders and hands are supposed to be doing in a sound mechanical swing, the likelihood of them dropping is decreased.
The hands should be moving BACK and around behind the head a bit up to the point that the front foot completes it's stride or, if you're using a minimal stride, when the heel drops.
One cue that seems to work is telling the hitter to "hide her hands" from the pitcher. From a front view the hands will indeed be hidden, as they rotate behind the head.
This "hand hiding" is a acomplished by shoulder rotation, or scapular loading. The rear shoulder will pinch inward toward the spine, while the front will pinch across the chest.
If the hands and shoulders are trained to do this, they simply cannot drop. It takes repetition and more repetition until these moves are ingrained into the hitter.
This type of loading will also increase bat speed, bat quickness and the power transfered to the ball.
- As you are doing your drill work, constantly give verbal reminders to enforce the proper hand & shoulder movement. As the hitter is waiting to hit, remind them "hands steady". As the hitter begins her stride, tell her "hide the hands".
There is no quick fix. Burning these moves into muscle memory will take practice, practice and more practice. If you are able to work with your hitter on these points, I would be quite curious for you to post again later and let us know what results you have seen.