The hairline cluster is OK for practice. The angle is tricky so for real self defense if my life depended on it, I'd just shoot for the eyes and call it a day. The hairline cluster kind of strikes down and doesn't work well for me in practice. For a take-down, I'd rather use third eye, a bit lower, rather than risk sliding around on a sweaty forehead too far up. With a baseball cap on with the bill forward, supposedly, you can hit the bill straight on, which would give you the downward angle for the hairline cluster strike. Dillman talks about it in one of his old videos, however, I've only heard of one person actually getting that KO for real. My coach says that when he was a young lad of about 8, a girl actually KO'd him with that strike to his hat.
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The older I get, the better I was!