The Master's Death Cart

I'm curious as to how some of it works. Obviously when you tag the character in, multiple ticks of damage applies to the enemy which also charges up your power bar.

Now here is what I don't understand about it: 1. At random moments after using my 2 power bar attack (idk what it's called), my power bar instantly fills back up (happens rarely)

  1. On combo ender, I deal a bit of damage to the opponents tagged out characters. Usually it ends up being a bunch of small amounts like 400,50,800,200 etc. But on rate occasion, I end up dealing ticks of 5000,3500 etc. Sometimes even 10 000.

Any explanation to help me better understand this? The character I use this on is Killer Frost and the other gear I use on her is Icebound Necklace and Demon Blade.