Einhar's Beastrite

Ok, so how is this lineage support gem actually supposed to work?

I kill a rare with low life and then I get one icon that says I have one or two modifiers via "Behead". So far so good, but...

1. How am I supposed to know what modifiers I have and what they do? Why are there no individual symbols telling me that?

2. The description says I get 2 modifiers per rare I kill, but as soon as I kill another rare, the modifiers get replaced and not added to the pool.

3. If I kill non-rare monsters with that skill, the modifiers disappear.