The tilt is tiny, all of 3 degrees, and the net effect is very satisfying -- you gradually add snacks to the "light" side until it makes a soft and very definite *click* as it falls.
This kind of 'very mild persuasion' example is a great demonstration of how a simple physical property can be used to inform the user - the conventional modern solution in this area might be to monitor users' behaviour, e.g. by weighing the amount of food put into the bowl, and then display it electronically, with an indication of whether a pre-set portion size has been exceeded. But these bowls simply tilt, with no electronics or moving parts (other than the bowl itself) necessary. It's an elegant poka-yoke style solution.
Portion perception (and unit bias) is a fascinating area - we've looked briefly at it a few times - but I hope to explore it in more detail in due course, along with a review of Dr Brian Wansink's Mindless Eating - in a post about how cognitive biases could be used in designing behavioural change.