You guys will not convince me otherwise: they are making a mountain out of a molehill with this POE. They are causing a problem that just is not there...just like MLB in 1988.
I have attended our state pre-season clinic given by the state assistant commissioner for baseball (a non-baseball person...a nice guy, but a non-baseball person). They have given us our marching orders: balk it (or ball it), or else.
This has never, ever been an issue in over a decade of doing FED ball. I have never heard of the term "hybrid stance" until FED came up with it this year. I've never had a pitcher take to the mound where I have not been able to tell which stance he was using. I've never once even come close to have an argument with an offensive coach who argued that a pitcher was using the opposite stance than the one I adjudged the pitcher to be using. Not once. I have not even had an argument where a coach argued that a pitcher was using an illegal stance and gaining an advantage (such as confusing the runners).
But with this POE AND the accompanying diagrams of pitchers' feet location (in both the NFHS pre-season guide and the NFHS overhead slides that were shown to all the umpires and coaches at our state pre-season clinic) they have made clear that what has been allowed (if not by rule, by fact) without incident for GOD knows how long in pro ball, NCAA and, even, FED will now be a balk or ball in FED.
As depicted in a picture from another thread, I have never once had any manager, coach or player on any level ever argue that a pitcher using the stance shown in this picture is pitching from anything other than the wind-up.
NFHS Rules: Pitcher's Stances.
Like in 1988, sometimes it is best just to let sleeping dogs lie. As President of our association I have already fielded calls from persons complaining about our umpires calling balls and balks in enforcing this rule...and these have only occurred in scrimmages. Our season doesn't start until this Saturday.
P.S. I'm not a fan of the "warn them early in the season and then call it when it gets to tournament (or playoff) time." I strive for consistency from myself and my association's umpires for the entire season. Unless they actually change the rules during the season...what is called and enforced in late February will be called and enforced the same exact way by our umpires at the State Championships in May.