Hobbs meters are electric, and have circuit breakers which are too easily pulled.
There was a famous case in Oz many years ago where a private 206 owner felt that the school using his machine on cross-hire was under-paying him. He installed a secret meter in the broom cupboard, and caught them out very badly - they had been pulling the breaker on the obvious meter, not recording the flight time on the MR and not paying the owner.
CASA gave them a kick in the slats, the CP took a jump but the company now known as The Breaker Brothers is still in business making obscene amounts of money.