I thought of it like when she gets up from a prone position, she is decreasing the blood going to her heart, so so the the myxoma causes more obstruction. Also myxomas cause tiny emboli, which can go to the systemic circulation causing mini thrombi in the brain and limbs.

Murmurs and maneuvers: 1st thought = how does it change with preload. All murmurs except HOCM, MVP, and atrial myxoma severity is directly proportional to change in preload (i.e. increased preload=worse murmur, etc.). Because of this, DDx can be narrowed down to HOCM, MVP, and atrial myxoma right away because the murmur worsened with decreased preload (i.e. standing up) when all but exceptions with improve.

Atrial myxoma = MCC primary cardiac tumor due to proliferation of connective tissue mesenchyme; a pedunculated mass connected via stalk to atrium septum that is suspended in the atrial blood volume and moves with the volume movement.

Presentation: triad of 1) mitral valve obstruction (i.e. malaise, symptoms of cardiac failure, syncope, etc.), 2) symptoms of embolism (i.e. facial and right arm hemiparesis in patient), and 3) constitutional symptoms (i.e. fever, weight loss, symptoms resembling connective tissue disease, because tumor releases IL-6). Others include neurologic symptoms, "pseudo-mitral valve disease" auscultatory findings (i.e. diastolic murmur), and atrial enlargement (which could compress underlying structures and cause symptoms also).

Not only does standing decrease preload, which means LA volume is lower so mass isn't as "suspended" but more mobile, standing also increases the downward gravitation force, which would contribute to the tumor moving towards the base of the atrial chamber, "plopping" on the mitral valve leaflets, and potentially extending through and causing a functional type of mitral stenosis (i.e. worsening diastolic murmur). This video explains it really well: https://www.youtube.com/watch?v=slIY64nViLg&t=161s