Why straining redirects bloodflow upwards?
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Blood flows from the heart through the body by the physical pressure exerted by the heart. Once the blood passes through the smaller vessels and capillaries it has barely any pressure behind it and it has to flow back to the heart through the veins. Simply explained, it’s the contraction and tension of the leg muscles that compress the veins of the legs that push the blood back upwards to the heart. The veins that go towards the heart have valve-like structures that prevent blood to flow back down to the legs.
There’s other forces at work that make the blood flow upwards like the negative (suction) provided by the right side of the heart, but the above is mostly why exertion and muscle tension causes an improvement of bloodflow back to the heart. It’s also the reason why jet fighter uniforms are pressurized. The added pressure makes the blood able to flow back to the heart and brain even under high G loads.
Many of your veins and small arteries are placed in or near your muscles in a way that let muscular action squeeze them. Especially in your limbs. By flexing all of the muscles you can manage to control at once, you squeeze a considerable amount of blood toward areas that *aren’t* affected by this action. Some of your torso and your head.
Does it redirect blood flow upwards, or “just” prevent it from flowing down?
(I’m thinking of the “strong core muscles, suck in your gut” technique used by Soviet/Russian pilots to prevent loss of consciousness during “high G” maneuvers in fighter jets.)
Google Valsalva Maneuver.
You hold you breath, close your glottis (shuts your throat, stops air from escaping from your lungs), and the pressure in your chest cavity drastically increases as you flex muscles which are trying to compress your chest. This resists the return of blood which was meant to be returning from the rest of your body (5L per minute, 80ml/sec very rough estimate) pools in the veins waiting to enter your chest – (SVC and IVC, google).
So 80ml/sec of blood pools in extrathoracic vessels while you hold your breath. Vessels in all parts of your body, including head and neck, accumulate fluid and engorge.
The physiology is much more complex and I’m not writing it out, but that’s the initial effect
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