In the video it is using my Rejuvenation buff instead of Miasma. In the actual WeakAura it is Miasma.
This WeakAura adds an icon for Miasma with text that shows your current health %. If you drop below 20k health it starts glowing. If you go over 95% health it stops glowing. When you drop under 20k it also starts saying "soak" over and over in the Dialog sound channel until you have more than 20k health. In order to hear the sound you have to have the addon "SharedMedia_Causese" installed. You can also choose your own sound under Condition 1 under the Conditions tab.