Hira-Ken-Tsuki is the leopard punch of Karate. It uses the middle knuckles instead of the knuckles to punch. In sport Kumite is usually not allowed, but in full-contact Kumite is one of my favourite punches to perform especially when there are more narrow spaces to hit. In self-defense it might be a best idea to stick to standard Sei-Ken punches (top two kuncles punches).
Traditional execution:
Kumite execution:
Where to use (✅ - Use; ❌ - Don’t use; 🟨 - Last resource):
Video examples: