"Flipping" is simply the result of applying leverage pressure to the golf shaft at the outset of the downswing. To NOT allow premature release and a waste of potential, stored energy in the 90 degree wristcock/fold position, DO NOT push the SHAFT in any possible way. Any pressure in your right hand that is happening DOES move the shaft early.
Do drills where you drag the GRIP CAP ALONE down and around in its arc, and pretend you are standing in a door jamb and need to hit the side of the entryway WITH the grip cap: not allowing ANY release, but hitting the door jamb with the clubshaft horizontal to the ground. Doing this with an IMAGINARY door jamb will inform you of your inability TO swing the club without release NECESSARILY happening. IOW, by trying to do something so extreme as that, you will discover, if you listen to your own feelings, just how you are shoving on the shaft inappropriately.
HKelley suggests for such a problem that you shorten your backswing for a while. It may help, but not nearly as much as UNDERSTANDING, and DEALING WITH, the root cause, which is that somehow in some subtle way, you ARE pushing on the shaft.
Another drill is to overlap your entire left hand with your entire right hand, no right hand fingers on the club at all. This way you CAN NOT shove the SHAFT with your right hand; you can only move the left hand. It is what HK refers to using the pressure point of the right wristbone; DO NOT use "pressure" in your right index finger, as that can well be the source of the problem of premature release.