http://forum.slowtwitch.com/...r%20alberto;#2249030 If you read the above thread you can see that not everyone defines "bonk" as complete glycogen depletion -but most of everyone does.
The "bonk" occurs at complete glycogen depletion, at which time your energy substrate for working muscle becomes almost entirely fatty acid metabolism via beta oxidation. Glucose is still produced via gluconeogenesis within the liver - to the level to support vital tissues aka the brain. You can only produce a limited amount of calories per unit time via beta oxidation which (probably among neuro-hormonal controls) forces you to throttle back and produces all of the symptoms.
If you have never had an overall lack of energy at the IM distance you most likely haven't experienced what everyone describes as the "bonk" and you are either really lucky, or have a gifted GI tract and metabolic system of a combination of both.
A good recipe to experience what everyone is describing would be to do a 3 day carb depletion while maintaining the intensity and duration of your workouts.
For safety I wouldn't suggest you ride the third day though.
Report back, because I'm pretty sure that you are missing out on something that 90 some odd percent of IM finishers have experienced - and we would like you to experience it too.
Kevin