Hmm... The rule is specified in the British Boxing Board of Control. Probably explains why they were more adament about it.
If it is illegal in the Nevada, I guess it would fall under the ref's discretion. If you put your head below an opponents waist, punches have nowhere to land but on the back of the opponents head/body. It's pretty much the same as turning your back against someone, which is usually done to stop an attack because blows to the back are illegal.
(h) persistently ducking below the waistline;

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