Unit Hit Point Bars

[Fargol]

During some games, in combat, I can see hit point bars for each troop in combat, both mine and enemy. The bar seems to represent the HP's of one unit within the stack (not the total remaining health of the stack).

It's on a hero-to-hero basis I don't know why I sometimes have them and sometimes not. I keep thinking it's a benefit of a particular skill. I also thought maybe I'm hitting an undocumented hot key.

Does anyone know what I'm talking about?