I think it probably means the sign of the cross, made with one's fingers.
You hold the extended forefinger of your left hand in a vertical position, and place the extended forefinger of your right hand across it in a horizontal position.
You direct the resulting cross towards the vampire in question, and it scuttles off to bury its fangs in some less dextrous mortal's jugular.
All the best,
PS: I imagine left-handed people hold their right forefinger vertically, and their left forefinger horizontally. It is no doubt just as efficacious.
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