Just to add to what Emily has posted; bonking usually includes nausea and dizziness when you get very depleted - it usually happens when you go beyond that ultimate fatigue. I have experienced it but not when lifting heavy - you just end up really tired and can't go any further. When you're doing circuit training or high volume training you lift lighter and it's very aerobic (a la CKD depletion workout pre carb up) - it's designed to deplete you completely and it leaves you not just shaky and tired, but feeling very close to heaving.

Nat