What does Clear Mind do?

[L O A F]

I just got all of the three things for Clear mind, but I'm not sure what it really does. I see a black bar near my health, but I don't know if I'm supposed to fill it or if it does anything. Does it unlock a secret ending?

[Velvet's Simp]

I do think it is explained if you hover the icon for it. But that extra black bar that showed up is extra energy for healing.
Edit: It should be an extra level of energy, so, one extra heal.