Why straining redirects bloodflow upwards?
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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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