It will only count when you "dominate" an enemy for the first time. When you hear that familiar sound that indicates a domination, it should increase your counter by 1.

Subsequent kills on a dominated enemy will not increase the counter for the part. However, that same player can remove the domination by:

• switching teams, then switching back
• leaving the server, then rejoining
• getting revenge

If any of these three things happen, the previous "domination" will be wiped, allowing you to dominate the player again, if you wish.

You only see the eye effect when the professional killstreak weapon is out. However, all weapons with killstreaks active count towards your eye effect. This means if I got 10 kills with my killstreak rocket launcher (no sheen, no eyes, just killstreak), then switch to my professional killstreak escape plan, you see the eye effect until I put my escape plan away.

The killstreak counter in the kill feed is based on that specific weapon, while the killstreak on your HUD is everything combined (all killstreak weapons; non killstreak weapons do not add to it). The sheen frequency and eye effect use the combined killstreak, meaning sheen frequency also carries over to other weapons, if you have multiple specialized killstreak weapons.

If you have multiple professional killstreak weapons with 5+ kills, switching between weapons simply swaps the eye effect according to the active weapon and its associated effect. They most definitely don't stack.