It’s not a random rush at all. Many times during the day your autonomous nervous system reacts without you consciously having to think about it – you breathe autonomously, you get hiccups autonomously, your intestines are told to move autonomously etc. Similarly, a stimulus you’re not even aware of might trigger the reaction. Or a “system check” happens – nerves that lead to you pelvic area are active and relax muscles that among other things control blood flow in your penis
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