I’m guessing they meant when they lean over, but the movement should be what does it. This movement uses gravity and the pooling of blood to force the liquid out of their sinuses - probably only temporarily though. Does it work the same way if they were to lean over the bed? If so, that’s the answer.

https://my.clevelandclinic.org/health/body/21778-nose

There is a number of sinus decongesting massage moves that work well. These use pressure and blood flow to force the liquid out of your sinuses and then uses gravity to take it away. I use these to help relieve sinus pressure.

https://health.clevelandclinic.org/sinus-massage/

But, to answer the original question, this is only related tangentially. Laying down, picking up your feet, and breathing generates a signal in your Vagus Nerve complex that essentially overrides the fainting signal. It’s still there, but you’ve added a new stronger signal to dominate the nerve and the fainting signal starts to fade. Simple meditative breathing does the same, but usually not fast or strong enough to stop you from syncope.

https://health.clevelandclinic.org/what-does-the-vagus-nerve...