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It's not really magic. There is a new type of transponder, that you manually switch between HOV and regular mode. Without this, you get charged a toll even if you would have qualified for HOV whether by plate or normal transponder.
We have the same concept here in Virginia. The enforcement is done by police---because the police have the electronics to see what mode the transponder is in. If it is in HOV mode, it is enforced the same was as a non-toll HOV lane violations. They police count the people in the car and issue an HOV violation.