Can you help me understand the relation between blood flow to the frontal cortex and thinking?

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For an example, having a normal chin and neck posture(relaxed one, not forcing it) increases the blood to your prefrontal cortex. The thing is that if you start to think on a hard problem your posture naturally starts to incline forward from the stress and it seems to result in less blood flow. It also seems that people with a good posture are not good thinkers yet it's like a paradox where you need the blood flow as studies show it correlates to higher IQ. Can there be a balance between less resources(inclining forward from stress) and proper blood flow?