How does a blood transfusion directly from one person to another through a line work?

Question

In Person of Interest, Shaw inserts a line into a guy's arm that goes directly into her arm for a blood transfusion. See clip: https://www.youtube.com/watch?v=eYQRa6IYf0A&t=83

What determines the direction the blood flows? How does she ensure it the blood flows to her rather than away? My guess is that because she's lost a lot of blood due to a gun shot, her blood pressure would be lower. Since his blood pressure would be higher, it pushes the blood to her accordingly. Once she receives a decent amount of blood, her pressure will increase until there's an equilibrium. Is that an accurate assumption or is there something else going on?

Answer 1

Direct blood transfusion is both dangerous for the donor and inconvenient in a modern medical setting, so this does not really happen today outside of movies/TV.

Before blood banking and anticoagulation, direct blood transfusion was done between the artery of a donor and vein of a recipient; pressure is much higher in the arteries than the veins. See Crile, 1907.

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Crile, G. (1907). I. The Technique of Direct Transfusion of Blood. Annals of surgery, 46(3), 329.