This may just be my interpretation, but the gist I got from the rules clinic that HLinNC posted was that the blocker must commit to the charge immediately after the snap, and the new rule effectively states that "immediately" means that the ball is still in the zone at that point, even if it is a shotgun snap. The key timing issue is when the block is *initiated*, not when it is completed. If there is any delay in initiating the low block, then it is not legal, regardless of whether the ball is still in the zone or not.
Later in the same video, he also makes the statement that stance is now *no longer a factor*. A player, on the line in the zone, in a two (or three or four) point stance may legally block below the waist, as long as it is an immediate initial action, and the player being blocked is also on the line in the zone at the snap. It is a foul if there is any delay in the blocking action or the person being blocked is not on the line and in the zone at the snap.
This rule change (appears to me, at least) seems to be designed to eliminate judgement calls and potential confusion regarding the foul, including trying to evaluate if the ball was still in the zone based on shotgun, stance, etc..