User blog comment:Xilinoc/Ch. 623 - Against The Slam/@comment-8669536-20150409130145/@comment-24099400-20150409134813

It's makes sense when you think about it, Underbelly definition - an area vulnerable to attack.

His analyzation of chakra points...oh I mean reiatsu allowing him to paralyze opponents leaving them vulnerable for him to capitalize on to attack lol.