Ascendancy: Arcane Surge?

Question: How does Arcane Surge granted by Ascendancy work?

Maybe this is common knowledge but I tried googling it and the only information I could find was related to the support gem and the condition that you spend your mana down to zero. So how does Arcane Surge granted by Ascendancy work? Do I also need to drop to zero mana to trigger it or is it and always on buff? If it is an always on buff should there be some buff indicator I should see somewhere indicating its in effect?

Please help!