these are two different weapons and will be used for different targets.
top of foot is faster, I think the act of pulling the top of the foot back slows the leg down a lot.
top of the foot is good for knee, peronial/t-band, sweeps, neck.
ball of foot is good for kidney, jaw, ribs, solar plexus, groin and hooking type sweeps or leg pics.
"George never did wake up. And, even all that talking didn't make death any easier...at least not for us. Maybe, in the end, all you can really hope for is that your last thought is a nice one...even if it's just about the taste of a nice cold beer."
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It's simpler than you think.
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