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There's a clinical name for deriving sexual pleasure from suffocation, strangling, or otherwise restricting the breath - it's called asphyxiaphilia. The practice is also known as "breath play" in the context of a sadomasochistic relationship, or auto-erotic asphyxiation when practiced alone in a masturbatory act. It's very dangerous - one of the most dangerous sexual kinks - and many people have died or suffered irreversible brain damage as a result of taking it too far.
Part of the reason that people find strangulation and breath play pleasurable is that it restricts the flow of oxygenated blood to the brain, and that can produce a state of euphoria and enhance sexual pleasure. Of course, those feelings are actually your body's way of signaling that your brain cells are dying and your body is going to quit too if you don't stop. Some people also get off on the feelings of danger and of being totally at the mercy of their partner.
If this scene really turns your girlfriend on, you can explore it through talking, fantasy, and role-playing. She can even hold her breath if she wants while you (lightly) touch her neck. Or you can find other, safer ways she can feel submissive to you. But for god's sake, don't put your hand around her neck and squeeze or otherwise block her airway. You're potentially setting yourself up for tragedy and criminal charges.
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