Why does one have to stay flat for 24 hours after vp shunt placement? afterwards, what happens when they elevate upright/lie back down?

Prevent overdrainage: The vp shunt is placed to drain fluid(csf) from the brain and reduce pressure.By lying flat the role of gravity which can increase the drainage of CSF and decompress the brain more rapidly than planned is eliminated .Shunts have valves to regulate the drainage but they are not perfect .Some vaves have anti-siphon device to prevent overdrainage while in the standing position.

Headache prevention: You can disobey, but you will likely be unhappy! after the rest and fluid adjustments one will be fine! some cerebrospinal, fluid is lost in the process of the surgery so the body needs to compensate.