u/Just-Chilling1234

New to the game and still learning (and will be for quite some time) but have a question my friends can’t answer. Playing Necromancer and I have the scythe ability, which is now in my primary slot. I notice during a battle there is a measuring bar that fills up on my first swing and then fluctuates as I fight. It shrinks to nothing when I stop. What does this mean? Level 23 player so that is why my question is so mundane in case you are wondering. Thank you for the help.