1. If you haven't seen it already check out Pauly's Whip Flip Drill with an emphasis on the last 20 seconds or so. Watch it several times.
2. The other is a quote from Sluggers and probably the biggest "light bulb" moment for us on this topic. Once I mentioned the "THUMB" and how it should look (which is the same in the whip flip (or at least my interpretation) my DD ran with it and hasn't looked back. Below is the quote and it is in the Basic arm-body synchronization sticky post #22.
Sluggers quote "Good IR involves rotation of the radius over the ulna. The easiest way to track the rotation is to look at the thumb. The thumb has to be on the third base side of the ball at release. For good IR, the thumb should be on the 1st base side of the ball when follow through completes. (Yes, I know that no forces are exerted on the ball after release. The follow through indicates whether the forearm was rotating at release.)"
For me, it was these 2 things that finally made it sink in. But I am a very slow learner.....
I hope it helps!