A Women's self-defense question: Most effective groin strike method?

Anonymous
My girlfriend asked me what's the most effective way to strike the groin. I wasn't sure; I haven't felt... all of it. But some girls who know about groin strikes may know. So, there's Kicking, Kneeing, Grab-Twist-Pull, Grab-Squeeze, Punch, etc. They teach about all these in my girlfriend's classes but didn't say which is most effective.
A Women's self-defense question: Most effective groin strike method?
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