The bar will begin to pulse when you are able to cast Void Eruption to enter Voidform. The green line denotes the amount of Insanity required to enter Voidform.
The bar will show different information once you are in Voidform. Seen in this screenshot include: Haste above a threshold (green text), VF stacks, time remaining, current drain rate, passive Insanity from Mindbender and Auspicious Spirits.
A more complicated bar text configuration.
Heavily customized bar layout showing the number of each DoT out on targets and icons for different sources of Insanity.
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